HHlHHI 


Hi 


LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 


Class 


I 


EXERCISES   IN    QUANTITATIVE 
CHEMISTEY 


BY 


HAEM01ST   NOETHKOP   MOKSE 

PROFESSOR  OF  ANALYTICAL  CHEMISTRY  IN  THE 
JOHNS  HOPKINS  UNIVERSITY 


GINN  &  COMPANY 

BOSTON  •  NEW  YORK  -  CHICAGO  •  LONDON 


ffr 


COPYRIGHT,  1905 
BY  HARMON  NORTHROP  MORSE 


ALL  BIGHTS  RESERVED 
55.7 


GINN   &   COMPANY  •   PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PREFACE 

The  essential  portions  of  this  book  have  been  in  use  in  the 
author's  laboratory  for  several  years  in  the  form  of  mimeo- 
graphed notes,  which  were  revised  and  expanded  from  time  to 
time  until  they  became  too  voluminous  for  further  duplication 
by  this  process.  Hence  they  have  been  put  in  book  form  for 
the  use  of  the  author's  students  and  of  any  others  who  may  find 
them  of  service. 

The  purpose  which  the  author  had  in  view  in  preparing  the 
notes  was  twofold.  First,  to  economize  the  energy  of  the 
student  by  placing  in  his  hands  a  working  guide  sufficiently 
precise  to  keep  him  in  the  right  path  during  the  absence  of  his 
teacher,  and  secondly,  to  increase  the  efficiency  of  the  teacher 
by  saving  him  waste  of  time  and  effort  in  repairing  the  grosser 
mistakes  of  inexperienced  students. 

The  book  is  not  intended  as  a  complete  guide,  even  in  the 
matters  contained  therein.  Enough  —  and  probably  more  than 
he  can  do  to  his  satisfaction  —  has  been  left  for  the  teacher, 
from  whom  the  student  may  justly  expect  at  all  times  the  warn- 
ing, suggestion,  or  amplification  which  will  contribute  most 
effectively  to  his  progress  and  discipline. 

The  problem  of  a  wise  choice  of  exercises  for  the  student  of 
quantitative  chemistry  is  a  difficult  one.  It  is  much  more  com- 
plex than  formerly,  when  the  greater  portion  of  the  student's 
time  in  the  laboratory  was  devoted,  as  a  matter  of  course,  to 
analytical  chemistry.  It  was  possible  then  to  train  expert 
analysts.  At  the  present  time  this  is  not  practicable,  except  at 
the  expense  of  other  work  which  is  everywhere  regarded  as 
indispensable.  The  chemical  student  in  training  for  his  calling 

iii 

194607 


iv  PREFACE 

is  now  required  to  take  somewhat  protracted  courses  of  experi- 
mental work,  which  are  arranged  with  a  view  to  familiarizing 
him  with  the  importantjreactions  of  substances,  and  that  with- 
out any  reference  to  the  utility  of  such  work  for  the  purposes 
of  analytical  chemistry.  To  this  end,  he  is  expected  to  prepare 
a  great  number  of  typical  compounds,  both  organic  and  inor- 
ganic. He  is  also  required  to  exercise  himself  in  the  methods 
peculiar  to  physical  chemistry.  Hence  the  time  which  can  be 
devoted  to  his  training  in  quantitative  chemistry  is  shorter  than 
formerly,  and  the  problem  of  what  to  give  him  to  do  in  this 
field  is  a  perplexing  one.  To  the  author  it  has  seemed  best  to 
select  the  limited  amount  of  work  which  the  student  can  do 
with  a  view  to  giving  him  a  familiarity  with  the  greatest  prac- 
ticable variety  of  quantitative  methods  and  operations,  and  not 
to  endeavor  to  train  him  especially  as  an  analyst.  This  book 
is,  therefore,  not  an  analytical  chemistry.  It  is  believed,  how- 
ever, that  a  student  who,  under  competent  guidance,  has  done 
well  the  work  which  is  here  prescribed  will  have  acquired  a 
skill  in  quantitative  manipulation  and  a  knowledge  of  methods 
which  will  enable  him  quickly  and  easily,  with  the  aid  of  the 
many  existing  excellent  works  of  reference,  to  become  proficient 
in  any  of  the  several  lines  of  quantitative  analytical  chemistry ; 
and  this,  it  appears  to  the  author,  is  the  end  to  be  sought,  and 
the  best  that  can  be  hoped  for  under  the  present  conditions. 

In  the  last  two  chapters  of  the  book  an  account  is  given  of 
certain  new  electrical  heating  appliances  for  laboratory  use, 
which  the  author  has  found  quite  serviceable;  also  of  a  new 
electrical  method  for  the  combustion  of  organic  compounds, 
which  in  his  laboratory  has  almost  entirely  supplanted  the 

older  process. 

HARMON  N.  MORSE 
MAT  1,  1905 


CONTENTS 

CHAPTER  I 
THE  BALANCE 

PAGE 

Description 1 

Precautions  to  be  observed  in  Weighing 8 

Exercise  I.    Practice  with  the  Balance       .         .         .                   .  10 

i.  Determination  of  the  Time  of  Vibration          .          .          .  10 

ii.  Determination  of  Zero  Points          .         .         .         .          .  11 

in.  Determination  of  Sensibility  .          .          .          .          .         .13 

iv.  To  determine  whether  the  Arms  are  Equal     .          .          .  16 

Exercise  II.    Practice  in  Weighing    .          .         .          .          .         .  19 

i.  Weighing  by  the  Usual  Method      .         .         .         .  19 

ir.  Weighing  by  the  Method  of  Borda          ."        .         .  20 

in.  Weighing  by  the  Method  of  Gauss           .         .         .         .  21 

iv.  Correction  for  the  Air  Displaced 22 

Weighing  by  Tares .  2'5 

Exercise  III.    Correction  of  Weights          .....  26 
i.  Comparison  of  the  Different  Pieces  of  a  Set  with  Each 

Other 26 

ii.  To  compare  a  Set  of  Weights  with  a  Standard  Weight  .  28 

Materials  for  Weights .         .         .  28 

CHAPTER  II 
THE  BAROMETER  AND  THE  THERMOMETER 

The  Barometer 30 

Corrections  of  the  Barometer 33 

i.  For  Temperature           ........  33 

ii.  For  Capillary  Depression      .......  35 

in.  For  Latitude  and  Altitude             ......  35 

iv.  For  the  Moisture  in  the  Air .  38 

The  Thermometer 38 

Exercise  IV.    Determination  of  the  Zero  Point          .         .         .  38 

Exercise    V.    Determination  of  the  Boiling  Point     ...  43 

Exercise  VI.    Calibration  of  the  Thermometer  45 


vi  CONTENTS 

PAGE 

Correction  for  the  Exposed  Part  of  a  Mercury  Column  ...  50 

The  Comparison  of  Thermometers 51 

Thermometers  filled  with  Gas         .......  51 

The  Beckmann  Thermometer         .......  52 

The  Air  Thermometer    .........  52 

Alcohol  and  Toluene  Thermometers       .         .         .         .                  .  54 

Determination  of  Temperatures  by  Substances  of  Known  Melting 

Points  54 


CHAPTER  III 

THE   CALIBRATION   OF  EUDIOMETERS  AND  THE 
MEASUREMENT  OF  GASES 

Exercise  VII.    Calibration  of  a  Eudiometer      ....  57 

i.  Purification  of  the  Mercury 57 

ii.  Calibration  of  the  Eudiometer 60 

in.  Determination  of  the  Value  of  the  Meniscus  ...  66 

iv.  Determination  of  the  Volume  of  the  Calibration  Unit    .  68 

Measurement  of  Gases  over  Water 69 

The  Absorption  of  Gases  by  Liquids       .         .         .         .         .         .  70 

The  Correction  of  Gas  Volumes  for  Pressure  ....  73 

The  Correction  of  Gas  Volumes  for  Temperature  .         .         .         .  74 

The  Correction  of  Gas  Volumes  for  Water  Vapor  .         .         .  75 

The  Passage  of  Gases  through  Rubber 77 


CHAPTER  IV 

CALIBRATION  AND  GRADUATION  OF  APPARATUS  FOR 
THE  MEASUREMENT  OF  LIQUIDS 

Exercise  VIII 79 

i.  Determination  of  the  Capacity  of  a  Measuring  Flask  by 

weighing  Water 79 

ii.  The  Graduation  of  a  Measuring  Flask    .         .         .         .  81 

in.  The  Calibration  of  Burettes  by  Means  of  Mercury .         .  83 

The  System  of  Mohr 83 

Exercise  IX.    The  Calibration  and  Graduation  of  Measuring 

Flasks  and  the  Calibration  of  Burettes         ...  85 

The  Graduation  of  Glass  Tubes      .         .         .         .         ..         .         .  94 

Standard  and  Normal  Solutions      .                                               .  99 

The  Correction  of  Standard  Solutions  for  Temperature  .         .         .  102 


CONTENTS  vii 


CHAPTER  V 

THE  PREPARATION  OF  STANDARD  SOLUTIONS  OF  ACIDS 
AND  ALKALIES 

PAGE 

Indicators 110 

1.  Litmus      . Ill 

2.  Phenolphthalein 113 

3.  Methyl  Orange 114 

4.  Tropseolin 115 

5.  Cochineal          .                 115 

Exercise  X.    The  Preparation  of  Standard  Solutions  of  Acids 

and  Alkalies 116 

i.  By  Means  of  Oxalic  Acid 116 

ii.  By  Means  of  Carbonates 123 

Other  Methods  of  standardizing  Acids  and  Alkalies       .         .         .  125 

1.  By  Means  of  Potassium  Tetroxalate           ....  126 

2.  By  Means  of  Sodium  Carbonate 127 

3.  By  Means  of  Sulphuric  Acid  derived  from  Copper  Sulphate  127 


CHAPTER  VI 
THE  DETERMINATION  OF  SPECIFIC  GRAVITY 

Density  and  Specific  Gravity 128 

Exercise  XI.    Determination  of  the  Specific  Gravity  of  Solids  .  130 
i.  Determination  of  the  Specific  Gravity  of  a  Silver  Coin    .  130 
ii.  Determination  of  the  Specific  Gravity  of  Glass  in  Frag- 
ments .         .         .         .         .         .         .         .         .         .  131 

Exercise  XII.    Determination  of  the  Specific  Gravity  of  Liquids  133 

i.  With  the  Pycnometer 133 

ii.  By  weighing  an  Object  in  Two  Liquids            .         .         .  134 

in.  By  the  Mohr-Westphal  Balance       .         .         .         .  135 

Hydrometers  (Areometers,  Densimeters) 136 

Class  A.    Instruments  of  Constant  Immersion 

1.  Fahrenheit's  Hydrometer           .         .         .         .         .         .  137 

2.  Nicholson's  Hydrometer   .......  138 

Class  B.    Instruments  of  Variable  Immersion 

1.  Gay-Lussac's  Volumeter 138 

2.  Specific-Gravity  Hydrometers    ......  140 

3.  Beaume's  Hydrometer       .         .         .         .         .         .         .  141 

4.  Beck's  Hydrometer 143 


viii  CONTENTS 

PAGE 

5.  Twaddell's  Hydrometer     .         .         .         .         .         .         .  143 

6.  The  Alcoholometer 144 

7.  The  Lactometer 144 

Specific-Gravity  Bulbs    .         .         .         .         .         .         .         .         .  145 

Determination  of  Specific  Gravity  by  Dilution        ....  145 

The  Density  of  Gases  —  Determination  of      .         .         .         .         .  146 

1.  By  weighing  the  Gases  directly 147 

2.  By  weighing  the  Gases  after  Absorption   ....  148 

3.  By  the  Time  required  for  Diffusion 149 


CHAPTER  VII 
THE  DETERMINATION  OF  MOLECULAR  WEIGHTS 

Atomic  Ratios         .         .         .         .         .         .    '               .         .         .  150 

1.  Copper  Oxide 150 

2.  Potassium  Permanganate 151 

3.  Acetic  Acid 151 

4.  Propyl-Amine    .         .         .         .         .         .         .         .         .  151 

5.  Benzene 152 

Molecular  Weights •      .         .         .  153 

The  Chemical  Method 153 

Physical  Methods 157 

i.  Dumas'  Method 158 

ii.  The  Method  of  Gay-Lussac 161 

Hofmann's  Method .  162 

Determination  of  the  Volume  of  a  Vapor  by  Means  of  the  Pres- 
sure which  it  Exerts    .         .         .         .         .         .         .  164 

Exercise  XIII.    Determination   of   the   Molecular  Weight  of 

Chloroform  by  the  Method  of  Meyer   .         .         .         .  165 

The  Freezing-Point  Method 169 

Exercise  XIV.    Determination  of  Molecular  Weights  by  the 

Freezing-Point  Method 177 

1.  Urea          .         .         .         ...         .         .         .         .       '  .  177 

2.  Cane  Sugar ,                .         .         .  179 

3.  Chloroform 179 

The  Boiling-Poiut  Method 179 

Exercise  XV.    Determination  of  Molecular  Weights  by  the 

Boiling-Point  Method          ....  .183 

1.  Iodine 183 

2.  Naphthalene 184 


CONTENTS  ix 


CHAPTER  VIII 
THE  PURIFICATION  OF  SUBSTANCES 

PAGE 

1.  Evaporation  of  Liquids      ...  186 

2.  Recrystallization         ....  190 

3.  Desiccation 

Hot-Air  Baths 194 

Liquid  Baths           ....  197 

4.  Precipitation      ........  198 

5.  Filters 200 

6.  Filtration 203 

7.  The  Washing  of  Precipitates 205 


CHAPTER  IX 
SILVER  AND  THE   HALOGENS 

Exercise  XVI.    Determination  of  Chlorine  and  Silver       .         .  209 

i.  Gravimetrically  as  Silver  Chloride 209 

ii.  Volumetrically  by  Mohr's  Method 216 

in.  Volumetrically  by  Volhard's  Method       .  218 

Exercise  XVII.    lodometric  Determinations      ....  222 

i.  Preparation  of  Standard  Solutions 223 

a.  Resublimed  Iodine 223 

b.  Potassium  Iodide 224 

c.  Starch 224 

d.  The  Solution  of  Iodine 224 

e.  The  Solution  of  Sodium  Thiosulphate     .         .  225 

f.  Standardization  of  Solutions  d  and  e  225 
ir.  lodometric  Determination  of  Sulphurous  Acid         .         .  226 

in.  lodometric  Determination  of  Chromic  Acid    .         .         .  228 

iv.  lodometric  Determination  of  Arsenious  Acid  .         .         .  229 

Exercise  XVIII.   Determination  of  Hypochlorous  Acid    .         .  229 

i.  By  Wagner's  Method 229 

ii.  By  Penot's  Method 230 

Exercise  XIX.    Determination  of  Halogens  in  Organic  Com- 
pounds   232 

i.  By  the  Lime  Method       .......  232 

ii.  By  Carius'  Method 234 

The  Separation  of  the  Halogens     .         .         .         .         .         .         .  236 

Exercise  XX.    The  Separation  of  Chlorine,  Bromine,  and  Iodine  239 


CONTENTS 

PAGE 

By  the  Method  of  Jannasch  and  Aschoif        .         .         .         .  239 

Determination  of  the  Iodine 240 

Determination  of  the  Bromine  ......  241 

Determination  of  the  Chlorine  .  241 


CHAPTER  X 
SULPHUR 

Exercise  XXI.    Determination  of  Sulphuric  Acid  in  Barium 

Sulphate 244 

Exercise  XXII.    Determination  of  Sulphur  in  Sulphides  .         .  247 
Exercise  XXIII.    Determination  of  Sulphur  in  Iron  by  Frese- 

nius'  Method       .         .         .         .         .         .         .         .  252 

Exercise  XXIV.    Determination  of  Sulphur  in  Organic  Com- 
pounds           256 

i.  By  Liebig's  Method 256 

ii.  By  Carius'  Method 257 

Hi.  By  Sauer's  Method 258 

CHAPTER  XI 
NITROGEN 

Exercise  XXV.    Determination  of  Nitric  Acid          .         .         .  261 

i.  By  the  Tiemann-Schulze  Method 261 

ii.  By  Crum's  Method 265 

Exercise  XXVI.    Colorimetric  Determination  of  Nitrous  Acid 

by  Method  of  Griess,  Preusse,  and  Tiernann         .         .  267 

Exercise  XXVII.    Colorimetric  Determination  of  Ammonia     .  270 
Determination  of  Free  and  Albuminoid  Ammonia  in  Water 

by  the  Method  of  Wanklyn,  Chapman,  and  Smith       .  270 
Exercise  XXVIII.    Determination  of  Nitrogen  in  Organic  Com- 
pounds        .........  272 

i.  By  Kjeldahl's  Method     .         .         ...         .         .         .  272 

a.  Wilfarth's  Modification 273 

b.  Gunning's  Modification  ......  274 

c.  Jodlbauer's  Modification 275 

d.  Foerster's  Modification   .         .         .         ..'..,         .  275 
ii.  By  Dumas'  Method 276 

in.  By  Varrentrapp  and  Will's  Method          ....  281 

Ruffle's  Modification   .  281 


CONTENTS  xi 

CHAPTER  XII 
PHOSPHORUS  AND  ARSENIC 

PAGE 

Exercise  XXIX.  The  Determination  of  Phosphoric  Acid  in 

Phosphate  Rock  .  . 283 

Exercise  XXX.  The  Determination  of  Phosphorus  in  Iron  by 

the  Stoeckmann-Fresenius  Method 287 

Exercise  XXXI.  The  Determination  of  Phosphoric  Acid  in 

Fertilizers 288 

a.  Determination  of  the  Water-Soluble  Phosphoric  Acid      .  289 

b.  Determination  of  the  Citrate-Insoluble  Phosphoric  Acid  290 

c.  Determination  of  Total  Phosphoric  Acid         .         .         .  291 

d.  Optional  Volumetric  Method  for  the  Determination  of 

Phosphoric  Acid  in  Fertilizers 292 

Other  Volumetric  Methods  for  the  Determination  of  Phosphoric 

Acid 293 

The  Determination  of  Phosphorus  in  Organic  Compounds     .         .  297 
The  Separation  of  Phosphoric  Acid  from  Other  Substances   .         .  298 
Exercise  XXXII.    The  Determination  of  Arsenic  as  Magne- 
sium Pyroarseniate 301 

CHAPTER  XIII 

SILICATES 

Silicates 303 

The  Determination  of  Water  in  Silicates 304 

1.  Sipcoecz's  Method 305 

2.  Jannasch's  Method 306 

3.  Chatard's  Method 307 

4.  The  Lead  Oxide  Method 307 

5.  The  Borax  Method 307 

Exercise  XXXIII.    The  Determination  of  Water  in  Silicates 

by  Means  of  Lead  Oxide 307 

Exercise  XXXIV.    The  Decomposition  of  a  Silicate  with  an  Al- 
kaline Carbonate  and  the  Determination  of  the  Silica  310 
Other  Methods  of  Decomposing  Silicates        .         .         .         .         .  312 

1.  Decomposition  by  Lead  Oxide 312 

2.  Decomposition  by  Bismuth  Oxide     .  314 

3.  Decomposition  by  Hydrochloric  Acid        .  314 

4.  Decomposition  by  Acids  under  Pressure  .         .         .         .  315 

5.  Decomposition  by  Hydrofluoric  Acid         .         .         .         .  316 


xii  CONTENTS 


6.  Decomposition  by  Barium  Hydroxide  and  Carbonate         .  317 

7.  Decomposition    by    Ammonium    Chloride    and    Calcium 

Carbonate 318 

Exercise  XXXV.    The  Determination  of  Silicon  in  Iron  .         .  318 

The  Separation  of  Silica  from  Other  Substances    .         .         .         .  319 


CHAPTER  XIV 
THE  DETERMINATION  OF  CARBONIC  ACID 

Exercise  XXXVI 320 

i.  The  Volumetric  Determination  of  Free  and  Semi-Com- 
bined Carbonic  Acid  in  Water    .         .         .         .         .  320 

Pettenkofer's  Method 320 

n.  The  Determination  of  Total  Carbonic  Acid  in  Water       .  323 
Exercise  XXXVII.   Determination  of  Carbonic  Acid  in  Car- 
bonates          326 

i.  By  Absorption  in  a  Weighed  Quantity  of  Soda-Lime       .   v  326 

ii.  The  Volumetric  Determination  of  Carbonic  Acid     .         .  328 

The  Separation  of  Carbonic  from  Other  Acids       ....  330 

Filtration  in  an  Atmosphere  Free  from  Carbonic  Acid  .         .         .  330 

Determination  of  Carbonic  Acid  in  the  Air    .         .         .         .         .  332 

The  Preparation  of  Carbonic  Acid  Gas  Free  from  Air    .         .         .  336 

CHAPTER  XV 

THE  DETERMINATION  OF  CARBON  AND  HYDROGEN  IN 
ORGANIC  COMPOUNDS 

The  Drying  of  Gases 339 

1.  Calcium  Chloride 339 

2.  Sulphuric  Acid 342 

3.  Phosphorus  Pentoxide 343 

4.  Anhydrous  Copper  Sulphate      .         .         .         .         .  345 

5.  Calcium  Oxide 345 

6.  Potassium  Hydroxide 345 

Materials  employed  in  the  Determination  of  Carbon  and  Hydrogen 

in  Organic  Compounds         ......  347 

1.  Copper  Oxide    : 347 

2.  Lead  Chromate- 349 

3.  Metallic  Copper 351 

4.  Oxygen 353 

5.  An  Apparatus  for  the  Purification  of  Oxygen  and  Air      .  353 


CONTENTS  xiii 

PAGE 

6.  An  Absorption  Apparatus  for  Water         .         .         .         .  354 

7.  An  Absorption  Apparatus  for  Carbon  Dioxide  .         .         .  355 

8.  Tubes   for   Protection    against  Atmospheric  Water  and 

Carbon  Dioxide 356 

Exercise  XXXVIII.    Determination  of  Carbon  and  Hydrogen  .  356 

i.  Combustion  of  a  Solid  in  the  Open  Tube        .         .         .  356 

ii.  Combustion  of  a  Liquid  in  the  Open  Tube     .         .         .  359 

in.  Combustion  in  the  Closed  Tube      .....  361 

iv.  Combustion  of  a  Compound  containing  Sulphur      .         .  363 

o.  Combustion  in  the  Open  Tube        ....  364 

b.  Combustion  in  the  Closed  Tube      .         .         .      p .  364 

v.  Combustion  of  a  Compound  containing  a  Halogen          .  364 


CHAPTER  XVI 

THE  DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL. 
THE  INCINERATION  OF  ORGANIC  SUBSTANCES.  THE 
DETERMINATION  OF  ORGANIC  MATTER  IN  WATER 

Exercise  XXXIX.    Determination  of  Total  Carbon  in  Iron  and 

Steel 366 

i.  By  Direct  Combustion  in  a  Current  of  Oxygen       .         .  366 

ii.  By  Oxidation  with  Chromic  Acid 368 

in.  By  Oxidation  of  the  Carbon  after  Removal  of  the  Iron  .  371 

Exercise  XL.    Determination  of  Graphitic  Carbon  in  Iron        .  372 

Exercise  XLI.    Determination  of  Combined  Carbon  in  Steel     .  373 

Colorimetric  Method  of  Eggertz .                  .         .         .  373 

Exercise  XLII.    Determination  of  the  Fuel  Value  of  Coal         .  375 

i.  Determination  of  Water 376 

ii.  Determination  of  the  Volatile  Combustible  Matter          .  376 

in.  Determination  of  Fixed  Carbon 377 

Incineration   ...........  377 

Determination  of  Organic  Matter  in  Water 380 

The  Method  of  Kubel 381 

The  Method  of  Schulze          .....  381 

The  Method  of  Tidy 382 

The  Method  of  Wolff,  Degener,  and  Hergfeld        ...  383 

The  Method  of  Dupre"  and  Hake  ....  384 

The  Method  of  Dittmar  and  Robinson 385 

1.  Determination  of  Organic  Carbon          .         .         .         .  385 

2.  Determination  of  Organic  Nitrogen       ....  385 
The  Method  of  Frankland  and  Armstrong    .         .                  .  386 

Determination  of  Organic  Carbon  and  Nitrogen         .         .  386 


xiv  CONTENTS 

CHAPTER  XVII 
GAS  ANALYSIS 

PAGE 

Exercise  XLIII.    Determination  of  Oxygen  in  the  Air     .         .  392 

i.  By  Explosion  with  Hydrogen 392 

ii.  By  Absorption  with  Phosphorus      .         .         .'•".'.         .  395 

in.  By  Absorption  with  Potassium  Pyrogallate     .         .         .  397 

Exercise  XL IV.    Determination  of  Hydrogen  .         .         .         .  398 

i.  By  Explosion  with  Oxygen 398 

n.%Jy  Absorption  with  Palladium 399 

Exercise  XLV.    The  Analysis  of  Illuminating  Gas   .         .         .  401 

Method  of  Hempel .401 

CHAPTER  XVIII 

THE  METALS  OF  THE  ALKALIES  AND  OF  THE  ALKALINE 

EARTHS 

Exercise  XL VI.    Determination  of  Potassium  as  Double  Potas- 
sium-Platinum Chloride       .         .         .         .         .         .  407 

The  Monroe  Filter 409 

Other  Methods  of  determining  Potassium 409 

Exercise  XL VII.    Separation  and  Determination  of  Potassium 

and  Sodium         .         .         .         .         .         .         .         .  410 

The  Separation  of  Potassium  and  Sodium  from  Other  Substances  411 

1.  Sulphates 412 

2.  Phosphates 412 

3.  Borates 413 

Exercise  XL VIII.   Determination  of  Potassium  and  Sodium  in 

a  Silicate 413 

The  Determination  of  an  Alkaline  Carbonate  in  a  Caustic  Alkali  415 
The  Determination  of  Acid  Carbonates  in  the  Presence  of  Neutral 

Carbonates 416 

Exercise  XLIX.    Separation   and   Determination   of   Barium, 

Strontium,  and  Calcium      ......  417 

By  the  Methods  recommended  by  Fresenius  .         .         .         .  417 

The  Detection  of  Barium,  Strontium,  and  Calcium         .         .         .  421 

The  Process  recommended  by  Fresenius         ....  421 

The  Quantitative  Determination  of  the  Individual  Alkaline  Earths  423 

1.  Barium      . 423 

2.  Strontium 427 

3.  Calcium    ,  429 


CONTEXTS  XV 

CHAPTER  XIX 
THE  ALKALINE  EARTHS  (continued)  AND  MAGNESIUM 

PAGE 

The  Separation  of  the  Alkaline  Earths  from  the  Acids           .         .  432 

1.  Sulphuric  Acid           .                            ...*..  432 

2.  Phosphoric  Acid        .  433 

3.  Hydrofluoric  Acid 434 

The  Separation  of  the  Alkaline  Earths  from  the  Alkalies       .          .  435 

The  Determination  of  Magnesium           ......  437 

The  Separation  of  Magnesium  from  the  Alkalies   .         .         .         .  438 

Other  Methods  of  separating  Magnesium  from  the  Alkalies  .         .  438 

The  Separation  of  Magnesium  and  Barium           •   .         .         .         .  440 

The  Separation  of  Magnesium  from  Strontium       ....  440 

The  Separation  of  Magnesium  from  Calcium          ....  441 

The  Separation  of  Magnesium  from  Barium,  Strontium,  and  Cal- 
cium   .         . 442 

The  Analysis  of   a  Solution    containing   the   Alkalies,   Alkaline 

Earths,  and  Magnesium      ......  443 

The  Precipitate 445 

The  Filtrate 446 

The  Hardness  of  Water 448 

Exercise  L.    Determination  of  the  Hardness  of  Water       .         .  450 

1.  Determination  of  Total  Hardness      .....  451 

2.  Determination  of  Permanent  Hardness     ....  452 

CHAPTER  XX 
POTASSIUM  PERMANGANATE 

Potassium  Permanganate 454 

1.  Oxalic  Acid 456 

2.  Potassium  Tetroxalate 459 

3.  Ferrous  Ammonium  Sulphate   ......  459 

Exercise  LI.    Determination  of  Iron  by  Means  of  Potassium 

Permanganate      ........  459 

i.  Preparation  of  the  Permanganate  Solution      .         .         .  459 

ii.  Determination  of  Metallic  Iron 461 

in.  Confirmation  of  the  Results  by  the  Gravimetric  Method  .  463 
Exercise  LIT.    Determination  of  Ferrous  and  Ferric  Iron  in  a 

Silicate 464 

Exercise  LIII.    Determination   and   Separation   of   Iron   and    ' 

Aluminium  466 


CONTENTS 


Exercise  LIV.    Determination  of  Manganese  by  Potassium  Per- 

manganate .........  468 

By  Volhard's  Method     .......  468 

Reduction  of  Permanganic  Acid  by  the  Oxides  of  Manganese        .  470 

Exercise  LV.    Determination  of  Manganese  as  Pyrophosphate  .  471 

By  Gibbs'  Method          ........  471 

Exercise  LVI.    Determination  of  Hydrogen  Peroxide  .     .         .  472 

Exercise  LVII.    Determination  of  Nitrous  Acid        .         .         .  472 


CHAPTER  XXI 
THE  ELECTROLYTIC  DETERMINATION  OF  METALS 

Electrical  Units  and  their  Relations        ......  474 

1.  The  Ampere  and  the  Coulomb  .......  474 

2.  The  Ohm  . 474 

3.  The  Volt 475 

4.  Ohm's  Law 476 

Determination  of  the  Strength  of  the  Current  .         .         .  476 

Determination  of  Voltage          ......  477 

5.  The  Joule 477 

6.  The  Watt           .                           .  478 

Sources  of  Current .  479 

Storage  Batteries    .         .  483 

Electrolysis 488 

1.  The  Electrodes 

2.  Rheostats 490 

3.  Faraday's  Laws 491 

4.  Polarization  Currents 493 

Exercise  LVIII.    Electrolytic  Determinations    ....  495 

1.  Copper 495 

a.  Classen's  Method       ...  495 

b.  Deposition  in  the  Presence  of  Nitric  Acid  .  496 

2.  Nickel 496 

Classen's  Method  ....  496 

3.  Iron 497 

Classen's  Method '      .  497 

4.  Zinc 498 

Classen's  Method  .                                     ....  498 


CONTP:NTS  xvii 

CHAPTER  XXII 
BUTTER 

PAGE 

Acids  occurring  in  Fats  and  Oils 500 

Exercise  L1X.    Determination  of  the  Insoluble  Acids        .         .  502 

Hehner's  Method 502 

Exercise  LX.    Determination  of  the  Volatile  Acids  .         .         .  504 
Exercise  LXI.    Determination  of  the  Alkali  required  for  Sapon- 

ification       .........  506 

Koettstorfer's  Method 506 

Exercise  LXII.    Determination  of  the  Alkali  neutralized  by  the 

Soluble  and  the  Insoluble  Acids 508 

Method  of  Morse  and  Burton         .  508 

Exercise  LXIII.    Determination  of  the  Iodine  Absorbed  .         .  508 

Htibl's  Method       ...                  .  508 

Exercise  LXIY.    Determination  of  Butter  in  Milk  .         .         .  512 

i.  Determination  by  the  Ordinary  Gravimetric  Method       .  512 

ii.  Determination  by  Adams' Method  .        ..         .         .  515 

in.  Determination  by  the  Method   of  Morse,  Piggot,  and 

Burton                           515 


CHAPTER  XXIII 

Electric  Heating  Appliances  for  Laboratory  Use     ....         517 

1.  Crucible  Furnace .         518 

2.  Air  Baths  with  Graphite  Stoves 524 

3.  Tube  Furnace 531 

4.  Platinum  Wire  Stoves  ....         533 


CHAPTER  XXIV 

An  Electrical  Method  for  the  Combustion  of  Organic  Compounds  537 

The  Combustion  of  Substances  containing  Nitrogen       .         .         .  543 

The  Combustion  of  Halogen  Compounds 544 

The  Combustion  of  Sulphur  Compounds 545 

Table  of  International  Atomic  Weights          .....  546 

INDEX  .  .  547 


LIST  OF  EXERCISES 


I.   Practice  with  the  Balance 10 

i.  Determination  of  the  Time  of  Vibration        ....       10 

ii.  Determination  of  Zero  Points         .         .         .         .         .         .       11 

in.  Determination  of  Sensibility          .         .         .         .         .         .       13 

iv.  To  determine  whether  the  Arms  are  Equal   .         .         .         .16 

II.   Practice  in  Weighing        ....  ....       19 


i.  Weighing  by  the  Usual  Method     . 

ii.  Weighing  by  the  Method  of  Borda 

in.  Weighing  by  the  Method  of  Gauss 

iv.  Correction  for  the  Air  Displaced  . 


19 
20 
21 
22 


III.  Correction  of  Weights      .                                   26 

i.  Comparison  of  the  Different  Pieces  of  a  Set  with  Each 

Other 26 

n.  To  compare  a  Set  of  Weights  with  a  Standard  Weight .        .  28 

IV.  Determination  of  the  Zero  Point  of  Thermometers  ....  38 
V.    Determination  of  the  Boiling  Point  of  Thermometers      ...  43 

VI.    Calibration  of  the  Thermometer       .......  45 

VII.    Calibration  of  a  Eudiometer    ...."....  67 

i.  Purification  of  the  Mercury 57 

ii.  Calibration  of  the  Eudiometer       ......  60 

in.  Determination  of  the  Value  of  the  Meniscus         ...  66 

iv.  Determination  of  the  Volume  of  the  Calibration  Unit  .         .  68 
VIII.          i.  Determination   of  the   Capacity  of  a  Measuring  Flask  by 

weighing  Water 79 

ii.  Graduation  of  a  Measuring  Flask 81 

in.  Calibration  of  Burettes  by  Means  of  Mercury        ...  83 
IX.    Calibration  and  Graduation  of  Measuring  Flasks  and  the  Calibra- 
tion of  Burettes .........  85 

X.   Preparation  of  Standard  Solutions  of  Acids  and  Alkalies         .         .116 

i.  By  Means  of  Oxalic  Acid 116 

ii.  By  Means  of  Carbonates        .......  123 

XL    Determination  of  the  Specific  Gravity  of  Solids       .         .         .         .130 

i.  Determination  of  the  Specific  Gravity  of  a  Silver  Coin          .  130 
ii.  Determination  of  the   Specific  Gravity  of    Glass  in  Frag- 
ments           131 

XII.    Determination  of  the  Specific  Gravity  of  Liquids     .         .         .         .133 

i.  With  the  Pycnometer 133 

n.  By  weighing  an  Object  in  Two  Liquids          .         .         .         .134 

in.  By  the  Mohr-Westphal  Balance 135 

XIII.  Determination   of  the  Molecular   Weight  of  Chloroform  by  the 

Method  of  Meyer 165 

XIV.  Determination  of  Molecular  Weights  by  the  Freezing-Point  Method  177 
XV.    Determination  of  Molecular  Weights  by  the  Boiling-Point  Method  .  183 

xviii 


LIST  OF  EXERCISES 


xix 


XVI.    Determination  of  Chlorine  and  Silver 209 

i.  Gravimetrically  as  Silver  Chloride 209 

ii.  Volumetrically  by  Mohr's  Method         ....  216 

in.  Volumetrically  by  Volhard's  Method    ....  218 

XVII.    lodometric  Determinations      .......  222 

i.  Preparation  of  Standard  Solutions        ....  223 

ii.  lodometric  Determination  of  Sulphurous  Acid      .         .  226 

in.  lodometric  Determination  of  Chromic  Acid.         .         .  228 

iv.  lodometric  Determination  of  Arsenious  Acid       .         .  229 

XVIII.    Determination  of  Hypochlorous  Acid      .....  229 

i.  By  Wagner's  Method 229 

ii.  By  Penot's  Method 230 

XIX.    Determination  of  Halogens  in  Organic  Compounds  .         .         .  232 
i.  By  the  Lime  Method     .         .         .         .         .         .         .232 

ii.  By  Carius'  Method 234 

XX.    Separation  of  Chlorine,  Bromine,  and  Iodine  ....  239 

XXI.    Determination  of  Sulphuric  Acid  in  Barium  Sulphate     .         .  244 
XXII.    Determination  of  Sulphur  in  Sulphides   .         .         .         .         .247 

XXIII.  Determination  of  Sulphur  in  Iron  by  Fresenius'  Method          .  252 

XXIV.  Determination  of  Sulphur  in  Organic  Compounds  .         .         .  256 

i.  By  Liebig's  Method 256 

ii.  By  Carius'  Method 257 

in.  By  Sauer's  Method .258 

XXV.    Determination  of  Nitric  Acid 261 

i.  By  the  Tiemann-Schulze  Method 261 

ii.  By  Crum's  Method 265 

XXVI.    Colorimetric  Determination  of  Nitrous  Acid  by  the  Method 

of  Griess,  Preusse,  and  Tiemann       ....  267 

XXVII.    Colorimetric  Determination  of  Ammonia         ....  270 

XXVIII.    Determination  of  Nitrogen  in  Organic  Compounds  .         .         .  272 

i.  By  KjeldahTs  Method 272 

ii.  By  Dumas'  Method 276 

in.  By  Varrentrapp  and  Will's  Method       .         .         .         .281 

XXIX.    Determination  of  Phosphoric  Acid  in  Phosphate  Rock    .         .  283 
XXX.    Determination  of  Phosphorus  in  Iron  by  the  Stoeckmann- 

Fresenius  Method 287 

XXXI.    Determination  of  Phosphoric  Acid  in  Fertilizers     .         .         .288 

XXXII.    Determination  of  Arsenic  as  Magnesium  Pyroarseniate  .         .  301 

XXXIII.  Determination  of  Water  in  Silicates  by  Means  of  Lead  Oxide  307 

XXXIV.  Decomposition  of  a  Silicate  with  an  Alkaline  Carbonate  and 

the  Determination  of  the  Silic'a          .         .         .         .310 

XXXV.    Determination  of  Silicon  in  Iron • .  318 

XXXVI.           i.  The  Volumetric  Determination  of  Free  and  Semi-Com- 
bined Carbonic  Acid  in  Water   .....  320 
n.  The  Determination  of  Total  Carbonic  Acid  in  Water   .  323 
XXXVII.    Determination  of  Carbonic  Acid  in  Carbonates        .         .         .  *  326 
i.  By  Absorption  in  a  Weighed  Quantity  of  Soda-Lime    .  326 
n.  The  Volumetric  Determination  of  Carbonic  Acid          .  328 

XXXVIII.    Determination  of  Carbon  and  Hydrogen 356 

i.  The  Combustion  of  a  Solid  in  the  Open  Tube         .         .  356 

n.  The  Combustion  of  a  Liquid  in  the  Open  Tube     .         .  359 

in.  Combustion  in  the  Closed  Tube    ,                                 .  361 


XX 


LIST  OF  EXEKCISES 


1'AGE 

iv.  The  Combustion  of  a  Compound  containing  Sulphur       .  363 

v.  The  Combustion  of  a  Compound  containing  a  Halogen  .  364 

XXXIX.    Determination  of  Total  Carbon  in  Iron  and  Steel       .         .         .  366 

i.  By  Direct  Combustion  in  a  Current  of  Oxygen        .         .  366 

ii.  By  Oxidation  with  Chromic  Acid 368 

in.  By  Oxidation  of  the  Carbon  after  Removal  of  the  Iron  .  371 

XL.    Determination  of  Graphitic  Carbon  in  Iron        ....  372 

XLI.    Determination  of  Combined  Carbon  in  Steel      ....  373 

XLII.    Determination  of  the  Fuel  Value  of  Coal 375 

i.  Determination  of  Water  .......  376 

ii.  Determination  of  the  Volatile  Combustible  Matter  .         .  376 

in.   Determination  of  Fixed  Carbon       .         .         .         .         .  377 

XLIII.    Determination  of  Oxygen  in  the  Air 392 

i.  By  Explosion  with  Hydrogen 392 

ii.  By  Absorption  with  Phosphorus 396 

in.  By  Absorption  with  Potassium  Pyrogallate      .        .         .  397 

XLIV.    Determination  of  Hydrogen 398 

i.  By  Explosion  with  Oxygen       ......  398 

ii.  By  Absorption  with  Palladium         .....  399 

XLV.    Analysis  of  Illuminating  Gas 401 

XLVI.    Determination    of    Potassium    as   Double   Potassium-Platinum 

Chloride 407 

XL VII.    Separation  and  Determination  of  Potassium  and  Sodium    .         .410 

XLVIII.    Determination  of  Potassium  and  Sodium  in  a  Silicate         .         .  413 
XLIX.    Separation    and    Determination    of    Barium,    Strontium,    and 

Calcium .        .         .  417 

L.    Determination  of  the  Hardness  of  Water 450 

LI.    Determination  of  Iron  by  Means  of  Potassium  Permanganate  .  459 

i.  Preparation  of  the  Permanganate  Solution       .         .         .  459 

ii.  Determination  of  Metallic  Iron        .....  461 

in.  Confirmation  of  the  Results  by  the  Gravimetric  Method  .  463 

LII.    Determination  of  Ferrous  and  Ferric  Iron  in  a  Silicate      .         .  464 

LIII.    Determination  and  Separation  of  Iron  and  Aluminium      .         .  466 

LIV.    Determination  of  Manganese  by  Potassium  Permanganate         .  468 

LV.    Determination  of  Manganese  as  Pyrophosphate          .         .         .471 

LVI.    Determination  of  Hydrogen  Peroxide         .....  472 

LVII.    Determination  of  Nitrous  Acid 472 

LVIII.    Electrolytic  Determinations       .  - 495 

LIX.    Determination  of  the  Insoluble  Acids  in  Butter          .         .         .  502 
LX.    Determination  of  the  Volatile  Acids  in  Butter  .         .         .         .504 
LXI.    Determination    of  the  Alkali   required  for   Saponification  of 

Butter 506 

LXII.    Determination  of  the  Alkali  neutralized  by  the  Soluble  and 

Insoluble  Acids  in  Butter 508 

LXIII.    Determination  of  the  Iodine  absorbed  by  Butter        .         .         .  510 

LXIV.    Determination  of  Butter  in  Milk 512 

i.  Determination  by  the  Ordinary  Gravimetric  Method       .  512 
ii.  Determination  by  Adams'  Method  .         .         .         .         .515 
in.  Determination  by  the  Method   of  Morse,  Piggot,  and 

Burton                                                                             .  515 


OF   THE 

f.   UNIVERSITY   ) 

OF 


QUANTITATIVE   EXERCISES 


CHAPTER  I 
THE   BALANCE 

The  object  to  be  accomplished  by  weighing  is  to  determine 
the  amount  of  matter  in  materials,  i.e.  their  mass;  and  the 
weights  which  are  used  in  the  process  are  to  be  regarded  merely 
as  standard  masses.  In  weighing,  that  is  in  comparing  objects 
with  the  weights  with  respect  to  their  mass,  the  force  of  gravity 
is  utilized.  This  force  varies  with  latitude  and  with  elevation 
above  sea  level ;  but  the  results  of  weighing  are  not  altered  by 
change  of  locality,  since  the  object  and  the  standard  with  which 
it  is  compared  are  affected  in  the  same  degree  by  the  change. 
It  is  only  in  the  weighing  of  gases,  when  the  relation  of  weight 
to  volume  is  to  be  determined,  that  the  fact  of  the  variable 
intensity  of  the  earth's  attraction  must  be  taken  into  considera- 
tion by  the  chemist. 

The  balance  used  by  chemists  in  determining  weight  is  a 
lever  whose  two  arms  are  of  equal  length  and  weight.  This 
lever  is  known  as  the  beam,  and  the  form  which  is  given  to  it 
is  that  which  is  supposed  to  combine  a  maximum  of  rigidity 
with  a  minimum  of  weight.  The  material  of  which  it  is  made 
is  usually  brass,  though  sometimes  an  alloy  of  aluminium  with 
other  metals  is  used. 

The  axis  about  which  the  beam  rotates  is  the  sharp  edge  of 
a  wedge-shaped  piece  of  agate  or  of  hardened  steel.  This  is 
known  as  the  central  knife  edge.  It  is  placed  at  right  angles  to 
the  vertical  plane  in  which  the  beam  is  to  move,  and  a  little 
above  the  center  of  gravity.  Hence  the  beam  can  move  in  but 

l 


2  QUANTITATIVE  EXERCISES 

one  plane,  and  it  always  tends,  when  left  alone,  to  take  a  hori- 
zontal position.  The  central  knife  edge  rests  upon  a  plate  of 
agate  which  is  fixed  in  a  horizontal  position  at  the  top  of  the 
standard. 

The  loads  —  the  object  to  be  weighed  and  the  weights  —  are 
suspended  from  the  two  ends  of  the  beam  at  equal  distances 
from  the  axis  of  rotation.  The  arrangement  for  the  suspension 
of  each  of  the  two  loads  consists  of  three  parts :  (1)  a  terminal 
knife  edge  of  agate  or  steel  which  is  fastened  to  the  beam  with 
its  sharp  edge  parallel  to  the  central  knife  edge  and  pointing 
upwards;  (2)  a  stirrup  holding  an  agate  plate  which  is  hung 
upon  the  knife  edge ;  and  (3)  a  pan  which  is  suspended  from 
the  stirrup  (usually)  by  means  of  a  jointed  hook.  Two  things 
are  essential  with  respect  to  the  position  of  the  loads :  (1)  the 
points  from  which  they  are  suspended  must  always  maintain, 
during  any  given  experiment,  the  same  relative  distance  from 
the  axis ;  (2)  the  point  of  suspension  and  the  center  of  gravity 
of  a  load  must  fall  in  the  same  vertical  line  whatever  the 
position  of  the  beam.  The  first  condition  is  fulfilled  by  placing 
the  terminal  knife  edges  parallel  to  the  central  one;  and  the 
second,  by  giving  the  pan  freedom  of  rotation  in  two  planes, 
one  of  which  is  the  plane  in  which  the  beam  moves,  and  the 
other  a  plane  at  right  angles  to  it,  i.e.  about  the  terminal 
knife  edge  and  on  the  jointed  hook.  A  second  knife  edge 
placed  in  the  stirrup  at  right  angles  to  the  terminal  one  upon 
the  beam  is  sometimes  substituted  for  the  joint  in  the  hook. 
Again,  in  some  balances  the  pan  is  suspended  from  the  stirrup 
by  means  of  a  chain  of  several  links. 

If  the  knife  edges  are  allowed  to  remain  continuously  in  con- 
tact with  their  bearings,  the  former  soon  become  dull,  and  fur- 
rows are  worn  into  the  latter,  rendering  the  balance  incapable 
of  accurate  weighing.  Every  instrument  is  therefore  provided 
with  a  device  called  the  arrest,  which  raises  the  beam  and  at 
the  same  time  lifts  the  stirrups  and  pans,  thus  separating  all 
three  of  the  knife  edges  from  their  bearings. 


THE  BALAKCE  3 

Owing  to  corrosion,  accumulation  of  dust,  or  unequal  expan- 
sion of  the  arms,  one  side  of  the  balance  (arm,  stirrup,  and  pan) 
sometimes  becomes  heavier  or  acquires  a  greater  leverage  than 
the  other,  so  that  the  beam  does  not  assume  a  horizontal  posi- 
tion in  a  state  of  rest.  To  remedy  this  difficulty  balances  are 
provided  with  an  arrangement  by  which  the  leverage  of  one  of 
the  arms  may  be  increased  or  diminished.  This  consists  some- 
times of  a  movable  vane  placed  above  the  center  of  the  beam, 
and  sometimes  of  a  nut  running  upon  a  screw  which  is  attached 
to  one  end  of  the  beam. 

To  determine  when  the  beam  is  in  a  horizontal  position  and 
also  to  enable  one  to  judge  its  movements  with  precision,  a 
long  pointer  is  attached  to  it  in  front  of  the  central  knife  edge. 
The  pointer  makes  right  angles  with  the  plane  of  the  three 
knife  edges,  and  its  lower  end  vibrates,  when  the  beam  is  in 
motion,  before  or  just  above  a  graduated  ivory  scale  which  is 
situated  at  the  base  of  the  standard. 

An  essential  feature  of  every  balance  is  the  device  by  which 
the  distance  between  the  center  of  gravity  of  the  beam  and  the 
axis  of  its  rotation  is  regulated.  This  consists  sometimes  of  an 
adjustable  bob  upon  the  pointer,  and  sometimes  of  a  screw  and 
nut  placed  vertically  above  the  center  of  the  beam. 

Finally,  every  balance  is  provided  with  a  spirit  level  or  a 
plumb  line  to  determine  when  the  plane  of  the  knife  edges  is 
horizontal,  and  with  a  case  to  protect  it  from  dust,  sudden 
changes  of  temperature,  and  draughts  of  air. 


A  balance  must  be  accurate  and  sufficiently  sensitive  to  meet 
the  requirements  of  the  work  to  be  done  with  it.  The  follow- 
ing are  the  more  important  of  the  conditions  which  affect  these 
qualities. 

The  Position  of  the  Center  of  Gravity.  In  order  that  the  beam 
may  always  tend  to  take  a  horizontal  position,  the  center  of 
gravity  of  the  balance  (beam,  stirrups,  and  pan)  together  with 


4  QUANTITATIVE  EXERCISES 

the  loads  upon  the  pans  must  lie  below  the  axis  of  rotation.  If 
the  center  of  gravity  were  in  the  axis,  the  equilibrium  would 
be  indifferent  and  the  beam  would  remain  in  any  position 
which  might  be  given  to  it.  If  it  were  above  the  axis,  the 
equilibrium  would  be  unstable  and  the  beam  once  moved  from 
its  horizontal  position  could  not  return  to  it. 

The  Distance  between  the  Center  of  Gravity  and  the  Axis. 
The  sensibility  of  the  balance  (which  is  the  angle  through 
which  the  beam  will  turn  for  a  given  difference  of  load  upon 
the  two  pans,  and  which,  therefore,  determines  the  precision 
with  which  objects  may  be  weighed)  depends  mainly  upon 
the  distance  between  the  center  of  gravity  and  the  axis  of  the 
beam's  rotation.  The  shorter  the  distance  between  them  the 
more  sensitive  is  the  balance.  Hence  the  utility  of  the  pro- 
vision which  is  made  in  the  construction  of  every  fine  balance 
for  the  raising  or  lowering  of  the  center  of  gravity  according  to 
the  fineness  of  the  work  which  is  required  of  it.  Of  the  two 
arrangements  for  this  purpose  already  referred  to  in  the  descrip- 
tion of  the  balance,  namely  the  threaded  post  and  nut  placed 
above  the  center  of  the  beam  and  the  movable  bob  attached  to 
the  pointer  by  means  of  a  set-screw,  the  former  is  much  more 
easily  managed,  and  is  therefore  to  be  preferred. 

As  the  distance  between  the  center  of  gravity  and  the  axis  is 
diminished  by  raising  the  former,  the  beam  vibrates  more  slowly. 
The  sensibility  of  any  given  balance  may  therefore  be  judged  by 
the  time  of  vibration  of  the  beam,  or,  what  is  the  same  thing, 
by  the  time  consumed  by  the  end  of  the  pointer  in  making  two 
successive  excursions  past  any  fixed  point  of  the  scale  at  the 
foot  of  the  standard. 

The  Knife  Edges  must  be  Parallel.  The  evil  consequences  of 
the  lack  of  parallelism  are  numerous,  and  they  vary  according 
to  the  character  of  the  defect.  It  will  suffice  to  mention  a 
single  possible  result,  namely  an  unequal  and  varying  dis- 
tribution of  the  burdens  along  the  knife  edges,  the  effect  of 
which  would  be  a  changeable  ratio  of  the  arm  lengths. 


THE  BALANCE  5 

The  Three  Knife  Edges  must  lie  in  the  Same  Plane.  This  is  a 
consequence  of  the  fact  that  the  loading  of  the  pans  (which  is 
in  effect  the  same  as  the  loading  of  the  terminal  knife  edges) 
changes  the  distance  between  the  center  of  gravity  and  the  axis, 
and  thereby  affects  the  sensibility  of  the  balance.  If  the  ter- 
minal knife  edges  are  below  the  central  one,  the  loading  of  the 
pans  lowers  the  center  of  gravity  and  diminishes  the  sensibility. 
If  they  are  in  the  same  plane  with  it,  or  above  it,  the  opposite 
effect  is  produced,  and  the  loading  of  the  pans  increases  the  sen- 
sibility. But  in  the  former  case,  i.e.  when  the  three  knife  edges 
are  in  the  same  plane,  the  center  of  gravity  can  never  reach  the 
axis;  hence  the  equilibrium  can  never  become  indifferent  or 
unstable.  In  the  construction  of  balances,  however,  it  is  per- 
missible to  elevate  the  plane  of  the  terminal  knife  edges  very 
slightly  above  the  central  one.  When  this  is  done,  it  is  for 
the  purpose  of  compensating  for  a  loss  of  sensibility  due  to 
bending  of  the  beam  and  to  friction  upon  the  knife  edges. 

The  bending  of  the  beam  diminishes  the  sensibility  by  lower- 
ing the  terminal  knife  edges ;  and,  since  no  beam  is  perfectly 
rigid,  the  effect  of  bending  upon  the  sensibility  is  usually 
noticed  when  a  balance  is  heavily  loaded.  It  will  be  seen  that 
the  extent  to  which  the  balance  maker  may  elevate  the  terminal 
knife  edges  above  the  central  one  is  limited  by  the  condition 
that  the  center  of  gravity  must  not  rise  quite  to  the  axis  when 
the  instrument  is  loaded  to  its  maximum  capacity. 

The  Length  and  Weight  of  the  Beam.  The  sensibility  of  the 
balance  depends  also  upon  the  length  and  weight  of  the  beam. 
The  longer  the  beam  the  greater  the  sensibility.  The  lighter  the 
beam  the  greater  the  sensibility.  Hence  the  rivalry  between 
the  long  and  the  short  beam  balances.  The  former  have  the 
advantage  of  greater  length  of  arm  and  the  latter  that  of 
lighter  weight.  The  short  beam  vibrates  more  quickly  than 
the  long  one,  and  this  fact  is  often  cited  as  its  principal  advan- 
tage, the  assumption  being  that  quick  vibration  saves  time  in 
weighing.  The  benefit  to  be  derived  from  this  property  of  the 


6  QUANTITATIVE  EXERCISES 

short  beam  is,  perhaps,  overestimated,  for  in  careful  weighing 
by  correct  methods  rapid  vibration  is  a  positive  disadvantage. 

The  Effect  of  Friction.  Friction  upon  the  knife  edges  dimin- 
ishes the  sensibility  of  the  balance  bj-  increasing  the  difficulty 
with  which  the  beam  and  the  pans  turn  upon  their  axes.  Any 
undue  friction  upon  the  terminal  knife  edges  may  also  prevent 
the  loads  from  acting  downward  in  vertical  lines,  and  thus,  in 
effect,  change  the  relative  lengths  of  the  arms.  Great  pains  is 
taken  in  the  construction  of  balances  to  reduce  the  friction  to  a 
minimum,  and  any  considerable  development  of  it  in  an  instru- 
ment should  be  investigated  at  once.  It  may  be  due  to  dust 
between  the  knife  edges  and  their  bearings,  to  the  rusting  of  the 
knife  edges  if  they  are  made  of  steel,  or  to  the  dullness  of 
the  knife  edges  and  the  furrowed  condition  of  the  bearings. 
The  last  conditions  develop  in  balances  which  are  carelessly 
used  or  left  to  swing  when  not  in  use,  and  also  in  those  which 
are  subjected  to  constant  jarring. 

Equality  of  Arm  Lengths.  Equality  in  the  length  of  the  two 
arms  is  desirable  but  not  essential,  since  for  most  purposes  it 
is  only  necessary  that  the  weights  to  be  determined  shall  be 
relatively  correct.  If  it  is  desired  to  obtain  correct  weighings, 
methods  are  employed  which  eliminate  the  errors  arising  from 
the  inequality  of  the  arms. 

The  Location  of  a  Balance.  A  balance  should  be  so  located 
that  the  two  arms  will  maintain,  as  nearly  as  possible,  the  same 
temperature  and  consequently  the  same  relative  length.  Hence 
it  should  not  be  placed  near  a  window,  a  door  which  is  fre- 
quently opened,  or  an  aperture  through  which  hot  or  cold 
air  enters  the  room.  Other  things  being  equal,  it  should  be 
placed  in  the  center  of  a  room  rather  than  at  the  side.  If 
the  light  is  sufficient,  it  is  well  to  protect  the  balance  from 
air  currents  by  surrounding  it  on  three  or  all  sides  by  curtains 
of  white  material.  A  balance  which  is  unfavorably  situated  as 
regards  sources  of  heat  or  cold  will  manifest  the  fact  by  con- 
stantly changing  its  zero  point,  i.e.  the  position  which  the 


THE  BALANCE  7 

pointer  would  take  before  the  scale  if  the  beam  should  come  to 
rest  after  having  been  set  in  motion.  If  artificial  light  is  used 
in  weighing,  the  lamp  or  burner  should  be  placed  above  and  back 
of  the  head  of  the  observer,  and  so  located  with  respect  to  the  beam 
that  the  heat  from  it  will  affect  both  arms  equally.  Whether 
the  light  is  properly  placed  or  not  can  be  ascertained  by  taking 
several  zero  points  in  succession,  —  that  is,  by  releasing  the  beam 
and  pans  at  short  intervals  and  determining  by  observing  its 
excursions  before  the  scale  where  the  pointer  will  rest  when 
the  beam  ceases  to  vibrate.  If  the  arms  are  unequally  heated 
by  the  lamp,  the  zero  points  will  vary,  tending  constantly 
toward  the  side  of  the  arm  which  is  less  rapidly  warmed. 

The  foundation  on  which  the  balance  rests  should  be  as  firm 
as  possible.  The  constant  jarring  of  a  balance  interferes  with 
weighing  by  rendering  the  correct  determination  of  zero  points 
difficult  or  impossible,  and  it  may  seriously  injure  the  knife 
edges  and  their  bearings.  The  extent  to  which  a  balance  is 
affected  by  the  unsteadiness  of  its  foundation  may  be  ascer- 
tained by  placing  a  thin  glass  bottle  partly  filled  with  mercury 
within  the  case  and  observing  the  movements  on  the  surface 
of  the  metal.  It  is  well,  if  practicable,  to  place  the  balance  on 
a  solid  pier  of  masonry  which  extends  some  distance  into  the 
earth,  and  whose  vertical  sides  do  not  come  into  contact  either 
with  the  earth  or  with  any  of  the  floors  of  the  building.  A 
heavy  wall  is  much  more  steady  than  a  light  one ;  hence  a  wall 
of  brick  or  stone  is  to  be  preferred  as  a  support  to  one  of  wood. 
A  balance  shelf  may  be  attached  to  such  walls  by  means  of 
heavy  iron  brackets,  but  in  no  case  should  the  shelf  be  partially 
supported  by  legs  resting  upon  the  floor.  The  effect  of  jarring 
upon  a  table  resting  on  the  floor  may  be  considerably  diminished 
by  placing  heavy  weights  upon  it.  An  arrangement  which  is 
said  to  be  effective  for  localities  where  it  is  otherwise  impos- 
sible to  secure  a  sufficiently  steady  foundation  for  balances  is 
that  which  was  first  used  in  the  provisional  office  of  the  K.  K. 
Normal-Aichungs  Commission  of  Austria.  An  air-tight  box 


8  QUANTITATIVE   EXERCISES 

capable  of  supporting  a  balance  in  a  bath  of  glycerin  is  sus- 
pended in  a  frame  by  diverging  chains  attached  to  its  four 
corners.  Underneath  this  is  placed  a  tank  of  glycerin.  The 
balance  is  so  located  upon  the  box  as  to  equalize  the  strain 
upon  the  chains,  and  glycerin  is  poured  into  the  tank  until  the 
box  with  its  burden  is  on  the  point  of  floating.  There  is  also 
an  arresting  device  operated  by  a  wheel  by  means  of  which  the 
box  and  balance  on  it  may  be  raised  and  made  steady  during 
the  introduction  and  removal  of  weights,  etc. 

Some  basic  substance,  such  as  lime  or  an  alkaline  carbonate, 
should  be  kept  in  the  balance  case  to  neutralize  acid  vapors. 
It  is  also  customary  to  place  in  the  case  some  drying  agent,  like 
calcium  chloride.  The  practice  is,  however,  of  doubtful  utility 
except  when  the  room  in  which  the  balance  is  kept  is  one 
of  unusual  humidity.  Strong  sulphuric  acid  should  not  be 
used  as  the  drying  agent,  since  the  dust  which  falls  into  it  may 
reduce  the  acid  with  formation  of  volatile  products  which  are 
injurious  to  the  balance. 

PRECAUTIONS  TO  BE  OBSERVED  IN  WEIGHING 

1.  Sit  directly  in  front  of  the  center  of  the  balance  so  as  to 
avoid  parallax  while  observing  the  movements  of  the  pointer. 

2.  See  that  the  balance  is  level. 

3.  Release  and  arrest  the  beam  and  pans  with  a  slow  and 
steady  movement  of  the  hand.     Jerky  movements  are  sure  to 
injure   the  knife   edges.     If  practicable,  the  beam  should  be 
arrested  only  when  it  is  in  a  horizontal  position.     The  pads,  or 
brushes,  under  the  pans,  which  are  designed  to  steady  them 
while   their  loads  are  being  placed   in   position  or   removed, 
should  be  lowered  before  releasing  and  raised  again  after  arrest- 
ing the  beam  and  pans. 

4.  Avoid  giving  to  the  pans  any  rotary  motion  in  a  horizontal 
direction,  and  all  other  movements  which  would  cause  a  knife 
edge  to  scrape  on  its  bearing. 


THE  BALANCE  9 

5.  Arrest  the  beam  and  pans  before  placing  anything  upon 
the  latter  or  removing  anything  from  them. 

6.  Place  the  object  to  be  weighed  and  the  larger  weights 
in  the  middle  of  the  pans.     If  the  terminal  knife  edges  are 
parallel  with  the  central  one,  and  if  there  is  no  considerable 
friction  either  upon  them  or  in  the  jointed  hook  or  chain  by 
which  the  pans  are  suspended  from  the  stirrups,  the  weighings 
will  be  correct  whatever  may  be  the  position  of  the  loads  upon 
the  pans.     The  above  precaution  is  nevertheless  to  be  observed, 
because  the  placing  of  a  heavy  load  at  a  distance  from  the 
center   causes   a   somewhat  violent  displacement  of  the  pan, 
which  is  followed  by  oscillations  not  easily  quieted. 

7.  See  that  the  rider  is  not  so  near  the  beam  as  to  be  hit  by 
it  while  swinging. 

8.  All  weighings  should  be  made  with   the   balance   case 
closed. 

9.  If  the  beam  does  not  begin  to  swing  as  soon  as  it  is 
released,  set  it  in  motion  by  wafting  the  air  over  one  of  the 
pans  with  the  hand,  or  by  raising  and  releasing  it  again. 

10.  Objects  cannot  be  correctly  weighed  while  hot,  owing  to 
the  upward  air  currents  which  they  create  about  the  pans  on 
which  they  rest.     They  may  also,  through  their  heating  effects 
on  the  beam,  produce  a  change  in  the  relative  lengths  of  the 
arms.     Neither  should  objects  be  weighed  while  they  are  colder 
than  the  atmosphere  of  the  balance  room,  since  in  that  con- 
dition they  may  condense  moisture  on  their  surfaces. 

11.  Hygroscopic  and  volatile  substances,  also   those   which 
absorb  carbon  dioxide  from  the  air,  must  be  weighed  in  closed 
vessels.     If  the  vessels  have  been  tightly  closed  while  warm, 
there  may  be  diminished  pressure  within,  in  which  case  they 
must  be  opened  for  a  moment  before  weighing. 

12.  Substances  which  are  exposed  to  the  air  condense  mois- 
ture on  their  surface  to  an  extent  which  sensibly  affects  their 
weight.     The  amount  of  moisture  thus  condensed  varies  with 
the  humidity  of  the  atmosphere ;  hence  a  substance  which  is 


10  QUANTITATIVE   EXERCISES 

transferred  from  a  desiccator  to  the  balance  pan  will  gain  in 
weight  for  a  time,  while  one  which  is  brought  from  a  damper 
atmosphere  than  that  within  the  balance  case  will  lose  weight. 
One  must  therefore  assure  himself,  before  regarding  an  observed 
weight  as  final,  that  the  object  is  neither  gaining  nor  losing 
from  this  cause.  Powdered  and  porous  substances  which  have 
been  dried  in  a  desiccator  or  in  a  hot-air  bath  should  be  weighed 
in  closed  vessels,  since  owing  to  the  great  surface  which  they 
present  to  the  air  they  condense  large  quantities  of  moisture. 
By  keeping  drying  agents  in  the  case  it  is  endeavored  to  main- 
tain a  fairly  uniform  condition  of  humidity,  and  thus  to  reduce  the 
errors  arising  from  the  condensation  of  water  upon  the  surface 
of  the  objects  weighed.  It  is  a  question,  however,  whether  the 
use  of  such  agents  does  not,  in  practice,  bring  about  greater 
fluctuations  of  humidity  within  the  balance  case  than  are 
observed  outside  of  it.  If  this  is  true,  it  would  manifestly 
be  of  advantage  to  discard  them  wherever  their  presence  is 
not  necessary  to  prevent  corrosion,  as,  for  instance,  of  steel 
knife  edges. 

13.  An  object  which,  like  glass,  is  likely  to  become  electri- 
fied by  friction  should  not  be  wiped  or  brushed  immediately 
before  weighing.     Glass  and  quartz  weights  sometimes  become 
strongly  electrified  when  lifted  out  of  their  places  in  the  box  in 
which  they  are  kept. 

14.  Long  tubes  and  other  objects  not  easily  centered  on  the 
pans  should  be  suspended  from  the  hooks  above  the  pans. 

EXERCISE  I 

PRACTICE   WITH    THE   BALANCE 
I.    DETERMINATION  OF  THE  TIME  OF  VIBRATION 

(a)  Dust  the  pans,  the  beam,  the  knife  edges,  and  their  bear- 
ings with  a  camel's-hair  brush.  Cautiously  release  first  the 
pans  and  then  the  beam.  After  the  beam  has  oscillated  a  few 


or  THE 

*E6?SITYniE  BALANCE  11 


times  —  long  enough  to  recover  from  the  effects  of  any  jar 
it  may  have  received  when  released  —  determine  the  time  con- 
sumed by  the  pointer  in  making  ten  excursions  past  the  central 
line  on  the  scale.  One  tenth  of  this  is  the  time  of  vibration. 

(b)  Repeat  the   experiment  with  long,   short,   and   medium 
excursions  of  the  pointer. 

(c)  Determine  the  time  of  vibration  with  loads  of  five,  ten, 
twenty,  thirty,  forty,  and  fifty  grams  upon  each  pan. 

The  time  of  vibration  will  be  found  to  vary  somewhat  with  the 
load.  It  also  varies  with  the  length  of  the  beam,  as  was  stated 
in  a  previous  paragraph ;  but  with  a  given  length  of  beam  and 
a  given  load  the  time  of  vibration  is  a  good  test  of  the  sensi- 
bility of  the  balance.  A  marked  decrease  in  time  of  vibration 
with  increased  loads  indicates  a  bending  of  the  beam. 

II.    DETERMINATION  OF  ZERO  POINTS 

(a)  Release  the  balance  and,  after  the  pointer  has  made  a  few 
excursions  over  the  scale,  begin  to  note  and  record  the  distances 
from  the  center  of  the  scale  at  which  the  pointer  stops  and 
begins  to  return,  estimating  to  tenths  of  a  division.  A  large 
magnifying  glass  may  be  used  with  advantage,  especially  if  the 
oscillations  are  rapid.  Place  the  readings  to  the  right  in  a 
column  marked  72,  and  those  to  the  left  in  another  column 
marked  L.  Take  a  number  of  observations,  say  three  or  four, 
on  one  side,  and  a  number  which  is  greater  or  less  by  one  on 
the  other.  Add  the  two  columns  and  divide  each  sum  by  the 
number  of  observations.  The  results  thus  obtained  represent 
the  mean  excursion  of  the  pointer  to  either  side  of  the  center 
of  the  scale.  Add  these  together,  divide  by  2,  and  subtract 
the  quotient  from  the  greater  of  the  two  mean  excursions.  The 
result  gives  the  distance  from  the  center  of  the  scale  —  on  the 
side  of  the  longer  swings  —  at  which  the  pointer  would  stop  if 
the  beam  were  to  come  to  rest,  i.e.  the  zero  point. 


12  QUANTITATIVE  EXERCISES 

Example 

L  E 

3.8  divisions  3.5  divisions 

3.6         "  3.3 

3.4        «  2)O 

3)10.8  3.4,  mean  excursion  E. 
3.6,  mean  excursion  L. 

Q    £*       I      O     A 

—  =  3.5,  one  half  of  the  total  mean  excursion,  which  taken 

2i 

from  3.6,  mean  excursion  to  the  left,  gives  the  zero  point  0.1. 
The  pointer  will  come  to  a  rest  one  tenth  of  one  division  to  the 
left  of  the  middle  line  on  the  ivory  scale.  It  will  be  seen  that, 
if  any  two  successive  readings,  as  L  3.8  and  E  3.5,  or  L  3.6  and 
E  3.3,  or  successive  pairs  of  readings,  as  L  3.8,  3.6  and  E  3.5, 
3.3,  had  been  treated  in  the  same  way,  the  zero  point  would 
have  been  located  at  0.15  of  a  division  to  the  left  instead  of  0.1. 
The  first  method  is  obviously  the  more  correct,  since  it  takes 
into  account  the  fact  that  the  excursions  become  shorter  and 
shorter  in  consequence  of  friction  on  the  knife  edges  and  the 
resistance  of  the  air. 

If  the  zero  point  is  found  to  be  more  than  one  half  of  one 
division  to  the  right  or  the  left  of  the  middle  line  on  the  scale, 
an  examination  should  be  made  to  ascertain  whether  the  dis- 
placement may  not  be  due  to  the  accumulation  of  dust  on  the 
beam  or  pans.  If  it  is  not,  —  that  is,  if  it  persists  after  a  careful 
dusting  of  the  balance,  —  it  is  probably  the  result  of  corrosion, 
and  the  zero  point  should  be  adjusted  to  its  proper  position  by 
means  of  the  arrangement  for  that  purpose  which  is  to  be  found 
on  every  balance.  Exact  adjustment,  however,  is  useless,  since, 
as  chemical  balances  are  usually  located,  the  relative  lengths 
of  the  arms  —  and  consequently  the  zero  points  —  are  constantly 
changing.  A  slight  displacement  of  the  zero  point  may  be  due 
to  the  fact  that  the  balance  is  not  level. 

(5)  Repeat  experiment  (a)  with  long,  short,  and  medium 
excursions  of  the  pointer.  The  zero  points  should  remain  nearly 
constant. 


THE  BALANCE  13 

(c)  Determine  the  zero  point  every  fifteen  minutes  for  an  hour. 

If  the  zero  points  are  found  to  vary  regularly  in  one  direction, 
or  for  a  time  in  one  direction  and  then  in  the  other,  it  is  prob- 
able that  the  ratio  of  the  arm  lengths  is  changing  in  consequence 
of  unequal  changes  of  temperature. 

Lack  of  constancy  in  a  series  of  zero  points  may  also  be  due 
to  dust  between  the  knife  edges  and  their  bearings,  to  the 
defective  condition  of  the  same  resulting  from  wear,  rust,  or 
imperfect  workmanship,  or  to  the  jarring  of  the  balance.  If  it 
is  due  to  dust,  wear,  or  to  rusted  knife  edges,  the  irregularity  will 
generally  be  found  to  be  in  some  way  related  to  the  length  of 
the  swings  employed  in  determining  the  points.  But  whether 
any  relation  of  this  kind  can  be  discovered  or  not,  the  working 
parts  of  the  balance  should  be  carefully  brushed  and  the  knife 
edges  and  their  bearings  should  be  examined  with  a  magnifying 
glass.  The  stability  of  the  foundation  on  which  the  balance 
rests  should  also  be  tested  by  placing  on  it  or  within  the  balance 
case  the  mercury  bottle  previously  mentioned.  Lack  of  con- 
stancy in  the  loaded  balance  may  be  due  to  the  unequal  bending 
of  the  two  arms. 


III.    DETERMINATION  OF  SENSIBILITY 

Strictly  speaking,  the  sensibility  of  a  balance  is  the  angle 
through  which  the  beam  will  turn  for  a  given  small  load  upon 
one  side  when  there  is  no  corresponding  counterpoise  upon 
the  other.  In  ordinary  parlance  it  is  the  displacement  of  the 
zero  point,  in  scale  divisions,  which  is  produced  by  adding  to 
or  subtracting  from  the  burden  borne  by  one  of  the  arms  a 
weight  of  one  milligram. 

(a)  Determine  the  zero  point  of  the  balance  without  load. 
Place  the  rider  on  the  one-milligram  division  of  the  graduated 
arm  and  again  determine  the  zero  point. 

The  distance  between  the  two  points  is  the  sensibility.  The 
true  sensibility,  i.e.  the  angular  displacement  of  the  beam 


14  QUANTITATIVE   EXERCISES 

produced  by  one  milligram,  can  be  calculated  if  the  length  of  the 
pointer  and  the  width  of  the  scale  divisions  are  known.  Other 
things  being  equal,  i.e.  length  of  beam,  weight  of  beam  and 
pans,  and  friction,  the  sensibility  depends  on  the  nearness  of 
the  center  of  gravity  to  the  beam  axis.  A  balance  is  sufficiently 
sensitive  for  ordinary  quantitative  work  when  the  addition,  of 
one  milligram  to  either  side  will  move  the  zero  point  from  two 
to  three  divisions  of  the  scale.  For  more  delicate  work,  such 
as  the  determination  of  atomic  weights,  the  center  of  gravity  is 
raised  until  the  same  difference  of  load  will  give  a  deflection  of 
six  or  seven  divisions.  With  a  sensibility  of  two  and  a  half 
or  three  divisions,  -fa  milligram  can  be  weighed  with  a  fair 
degree  of  certainty,  while  a  sensibility  of  five  or  six  divisions 
will  enable  one  to  detect  differences  of  weight  amounting  to 
not  more  than  ^  milligram.  The  adjustment  of  the  sensibility 
of  a  balance  should  not  be  undertaken  by  inexperienced  persons. 

(b)  Remove  the  rider,  place  five  grams  upon  each  pan,  and 
determine  the  zero  point.  Replace  the  rider  on  the  one-milli- 
gram division  of  the  arm  and  again  determine  the  zero  point. 
Repeat  the  experiment  with  loads  of  ten,  fifteen,  twenty,  twenty- 
five,  thirty,  forty,  and  fifty  grams  upon  each  pan. 

If  the  terminal  knife  edges  are  below  the  central  one,  the 
sensibility  will  rapidly  decline  with  increasing  loads.  Any 
bending  of  the  beam  will,  of  course,  produce  the  same  result. 
If  the  knife  edges  are  in  bad  condition,  the  sensibility  will  also 
decline  with  increasing  loads  in  consequence  of  undue  increase 
of  friction. 

If  the  three  knife  edges  are  in  the  same  plane,  or  if  the 
terminal  knife  edges  are  above  the  central  one,  the  center  of 
gravity  will  rise  when  the  loads  upon  the  pans  are  increased, 
and  the  balance  might  therefore  be  expected  to  exhibit  greater 
sensibility.  But  any  advantage  from  this  source  is  usually  some- 
what more  than  offset  by  the  loss  of  sensibility  which  results 
from  the  augmentation  of  the  mass  to  be  moved  in  the  opera- 
tion of  weighing,  i.e.  beam,  pans,  and  loads,  and  from  the 


THE  BALANCE  15 

bending  of  the  beam.  Hence,  as  a  matter  of  fact,  most  balances 
become  less  sensitive  when  heavier  loads  are  placed  upon  the 
pans.  If  a  balance  exhibits  a  reasonably  uniform  sensibility  up 
to  a  given  load  and  then  a  sharp  decline  for  greater  burdens,  it  is 
certain  that  the  beam  is  bending  under  the  heavier  weights,  and 
that  the  limit  to  which  it  ought  to  be  loaded  has  been  reached. 

There  are  two  methods  of  securing  constant  sensibility  in 
balances,  i.e.  a  sensibility  which  does  not  vary  with  the  weight 
to  be  determined.  One  of  them  was  first  employed  by  Verbeek 
and  Peckholdt  and  the  other  by  Bunge.  The  former  enhance 
the  increase  of  sensibility  resulting  from  the  rise  of  the  center 
of  gravity  when  additional  loads  are  placed  upon  the  pans  by 
fixing  the  terminal  knife  edges  above  the  central  one  and,  by 
exact  adjustment,  make  the  gain  from  this  source  just  equal  to 
the  loss  of  sensibility  from  all  other  causes.  The  adjustment 
must,  of  course,  be  such  that  the  maximum  load  for  any  balance 
can  neither  throw  the  terminal  knife  edges  below  the  central  one 
nor  raise  the  center  of  gravity  so  high  as  to  produce  indifferent 
equilibrium. 

In  Bunge's  balance  of  constant  sensibility  the  arms  are  of 
different  length  and  weight,  and  only  the  shorter  and  lighter 
one  is  supplied  with  a  pan.  This  difference  in  the  length  and 
weight  of  the  two  arms  is  so  regulated  that  the  maximum 
weight  which  the  balance  is  intended  to  carry,  say  200  grams, 
just  suffices  to  bring  the  beam  into  equilibrium.  The  object 
to  be  weighed  is  placed  upon  the  pan  and  weights  are  added 
until  the  balance  is  brought  to  a  state  of  equilibrium.  The 
difference  between  the  weight  which  is  known  to  produce 
equilibrium  and  the  sum  of  the  weights  placed  on  the  pan  is 
the  weight  of  the  object.  It  will  be  observed  that  the  balance 
always  carries  the  same  load,  and  that  for  this  reason  its  sen- 
sibility is  constant. 

Any  balance  of  ordinary  construction  can  be  used  as  an 
instrument  of  constant  sensibility  in  the  following  manner : 
A  tare  of  suitable  character,  which  is  heavier  than  the  heaviest 


16  QUANTITATIVE  EXERCISES 

object  to  be  weighed,  is  placed  upon  one  of  the  pans.  The 
object  to  be  weighed  is  placed  upon  the  other  pan  and  weights 
are  added  until  the  beam  is  in  equilibrium.  The  object  is  then 
removed  and  more  weights  are  added  until  the  beam  is  again 
in  equilibrium.  The  difference  between  the  weights  necessary 
to  produce  equilibrium  on  the  two  occasions  is  the  weight 
of  the  object.  This  method  of  weighing  with  the  ordinary 
balance  is  an  excellent  one,  since  it  offers  the  double  advan- 
tage of  constant  sensibility  and  elimination  of  errors  due  to  an 
unequal  or  variable  ratio  of  arm  lengths. 

The  distance  between  the  center  of  gravity  and  the  beam  axis 
varies  somewhat  with  the  temperature,  shortening  in  cold  and 
lengthening  in  warm  weather.  Hence  a  balance  with  a  given 
adjustment  is  found  to  be  more  sensitive  in  winter  than  in 
summer,  and  it  sometimes  happens  that  a.  balance  which  has 
been  adjusted  to  great  sensitiveness  in  hot  weather  exhibits 
indifferent  equilibrium  in  very  cold  weather. 

IV.     To    DETERMINE    WHETHER    THE    ARMS    ARE    EQUAL 

(a)  Determine  the  zero  point  of  the  empty  balance.     Place  a 
one-milligram  weight  on  the  right-hand  pan  and  determine  the 
zero  point.     Transfer  the  weight  to  the  other  pan  and  again 
determine  the  zero  point.     If  the  arms  are  of  the  same  length, 
the  last  zero  point  will  lie  as  much  to  the  right  as  the  second 
does  to  the  left  of  the  first. 

This  test  is  decisive  only  when  the  balance  is  very  sensitive 
and  the  knife  edges  and  their  bearings  are  in  good  condition. 

(b)  Determine  the  zero  point  of  the  empty  balance.     Place  a 
ten-gram  weight  upon  each  pan  and  determine  the  zero  point. 
Exchange  the  weights  and  again  determine  the  zero  point. 

If  the  arms  are  equal  and  the  weights  are  equal,  the  second 
and  third  zero  points  will  coincide  with  the  first.  If  the  arms 
are  equal  and  the  weights  unequal,  the  second  and  third  zero 
points  will  deviate  equally  to  the  right  and  left  of  the  first,  If  the 


THE  BALANCE  17 

arms  are  unequal  and  the  weights  equal,  the  second  and  third  zero 
points  will  coincide  with  each  other,  but  not  with  the  first.  If 
both  arms  and  weights  are  unequal,  the  second  and  third  zero 
points  cannot  coincide  with  each  other,  neither  can  they  deviate 
equally  to  the  right  and  left  of  the  first.  It  follows  that  if  the 
second  and  third  zero  points  coincide  with  the  first,  or  if  they 
deviate  equally  to  the  right  and  left  of  it,  the  arms  are  of 
equal  length.  Any  other  relation  of  the  three  points  indicates 
inequality.  Of  course  any  slight  difference  in  the  length  of 
the  arms  may  escape  detection  by  this,  as  by  all  other  methods, 
if  the  knife  edges  or  their  bearings  are  in  bad  condition. 

As  has  been  stated  already,  the  exact  ratio  of  the  arm  lengths 
is  of  little  concern  to  the  chemist,  since,  whatever  the  ratio 
may  be,  if  it  remains  constant,  his  weighings  are  relatively 
correct,  and  this  suffices  for  most  purposes.  The  ratio,  if 
known,  may  be  utilized  to  correct  weighings  ;  but  as  a  matter 
of  fact  it  is  not  so  used,  because,  first,  being  changeable,  it 
must  be  redetermined  for  every  new  experiment;  and,  second, 
there  are  easy  methods  of  weighing  which  eliminate  the  errors 
resulting  from  the  inequality  of  the  arm  lengths. 

The  ratio  is  ascertained  by  weighing  an  object,  usually  a 
brass  weight,  first  upon  one  pan  and  then  upon  the  other. 
Any  apparent  difference  of  weight  found  by  the  two  operations 
is  due  to  the  inequality  of  the  arms.  The  procedure  is  as 
follows. 

Determine  the  zero  point  of  the  empty  balance.  Place  a 
ten-gram  weight  upon  each  pan,  representing  them  by  P  and  P' 
respectively.  Determine  the  zero  point  and  the  sensibility  of 
the  balance  with  the  weights  upon  the  pans,  and  calculate  what 
weight  must  be  added  to  or  subtracted  from  P  to  produce 
equilibrium,  i.e.  to  make  the  zero  point  coincide  with  that  of 
the  empty  balance.  Exchange  P  and  P',  and,  in  the  same 
manner  as  before,  find  how  much,  plus  or  minus,  must  be  added 
to  P  to  produce  equilibrium.  Let  p  represent  the  weight  which 
must  be  added  when  P  is  on  the  left  pan,  and  p'  that  which 


18 


QUANTITATIVE  EXERCISES 


must  be  added  when  P  is  on  the  right  pan.     Let  It  and  L  rep- 
resent the  length  of  the  right  and  left  arm  respectively.     Then 

L  (P  +  p}  =  P'R, 
and  LP*  =  (P  +  /)  P. 

Multiplying  together  the  corresponding  terms, 
L*P'  (P  +  p)  =  R*(P  +  /)P'. 

Dividing  by  P',     i2  (P  +  p)  =  R*  (P  +  /), 

Z         /P 


or 


Dividing  numerator  and  denominator  of  the  second  term  by  P, 


Extracting  the  square  root  of  numerator  and  denominator 
separately,  we  have,  approximately, 


L 
& 


1  + 


•+£ 


Dividing  numerator  by  denominator, 


2P 


THE  BALANCE  19 

EXERCISE  II 

PRACTICE   IN   WEIGHING 
I.  WEIGHING  BY  THE  USUAL  METHOD 

(a)  Determine  the  zero  point  of  the  empty  balance.     Place 
the  object  to  be  weighed  —  a  crucible,  coin,  or  something  else 
which  does  not  change  its  weight  in  the  air  —  in  the  center  of 
the  left-hand  pan,  and  the  weights  which  are  supposed  to  be 
about  equal  to  it  in  the  center  of  the  right-hand  pan.     Slowly 
release  the  balance  —  pans  first,  and  then  the  beam  —  until  it  is 
seen  which  way  the  pointer  turns,  and  then,  as  slowly,  arrest  it 
again.     Add  or  remove  weights,  as  may  be  required,  and  again 
release  and  arrest  the  beam  as  before.     Repeat  these  trials, 
using  the  rider  for  weights  under  a  centigram  until  the  zero 
point  is  found  to  coincide  with  that  of  the  empty  balance. 

(b)  Proceed  as  directed  under  (a)  until' it  is  found  that  not 
more  than  one  milligram  must  be  added  to  or  taken  from  the 
weights  in  order  to  produce  equilibrium,  and  then  determine 
the  zero  point.     Add  or  subtract  one  milligram  by  moving  the 
rider  along  the  beam  and  again  determine  the  zero  point. 

The  last  operation  gives  the  sensibility  of  the  balance  for 
the  load  which  is  upon  the  pans,  and  from  this  is  to  be  calcu- 
lated what  weight  is  to  be  added  or  subtracted  in  order  to 
produce  equilibrium. 

Example 

Suppose  the  zero  point  of  the  empty  balance  is  0.5  division 
to  the  left  of  the  middle  line  on  the  scale,  and  the  zero  point 
when  the  weights  nearly  balance  the  object  is  1.0  to  the  right, 
while  the  sensibility  is  3.0  divisions.  It  is  clear  that  the  addi- 
tion of  one  half  milligram  to  the  weights  already  in  the  balance 
will  produce  equilibrium,  since  that  is  the  weight  required  to 
move  the  zero  point  1.5  divisions,  i.e.  from  1.0  right  to  0.5  left. 
Of  the  two  methods,  (a)  and  (&),  the  latter  is  the  better,  and 
it  should  be  generally  employed  in  quantitative  work. 


20  QUANTITATIVE  EXERCISES 

Instead  of  determining  the  sensibility  of  the  balance  each  time 
a  weighing  is  made,  one  may  utilize,  in  the  form  of  a  table  or 
a  curve,  the  sensibilities  for  different  loads  as  determined  in 
Exercise  I,  III.  This  course,  however,  is  not  to  be  recommended, 
since  it  is  not  safe  to  assume  that  the  sensibility  has  not 
changed  in  the  meantime.  It  will  be  seen  that  balances  of 
constant  sensibility  have  this  distinct  advantage  over  others, 
namely,  that  all  weights  less  than  a  milligram  can  be  read 
directly  from  the  scale. 

It  is  assumed,  whenever  either  of  the  methods  (a)  or  (b)  is 
employed,  that  the  arms  are  of  equal  length,  whereas  they  are 
usually  to  some  slight  extent  unequal.  But  no  harm  results 
from  this  assumption  if,  as  in  ordinary  analytical  work,  it  is 
only  necessary  that  the  various  weighings  shall  be  correct  with 
respect  to  each  other ;  and  they  are  thus  relatively  correct,  how- 
ever unequal  the  arms  may  be,  provided  the  ratio  of  the  arm 
lengths  does  not  change.  If  the  ratio  of  the  arm  lengths  were 
known,  the  errors  in  weighing  due  to  inequality  could  be  cor- 
rected; but  corrections  of  this  sort  are  seldom  undertaken 
because  the  ratio  is  not  constant.  It  varies  in  consequence  of 
unequal  changes  of  temperature  and  also  because  of  unequal 
bending  under  the  strain  of  the  loads  upon  the  pans.  If  the 
work  to  be  done  is  of  such  a  character  that  the  errors  in  weigh- 
ing due  to  inequality  or  varying  ratio  of  the  arm  lengths  cannot 
be  neglected  (i.e.  if  exact  weighings  are  required  rather  than 
those  which  are  correct  with  respect  to  each  other),  one  of  the 
two  following  methods  is  employed.  Both  eliminate  the  errors 
due  to  the  inequality  and  to  the  changeable  ratio  of  the  arms. 

II.  WEIGHING  BY  THE  METHOD  OF  BORDA 

(a)  Determine  the  zero  point  of  the  empty  balance.  Place 
the  object  to  be  weighed  upon  the  right-hand  pan  and  balance 
it  to  within  one  milligram  by  placing  weights  upon  the  left- 
hand  pan.  Determine  the  zero  point  and  the  sensibility,  and 


THE   BALANCE  21 

from  these  calculate  how  much  would  have  to  be  added  to  or 
subtracted  from  the  weight  of  the  object  to  produce  equilibrium, 
i.e.  to  make  the  zero  point  coincide  with  that  of  the  empty 
balance.  Remove  the  object  and  put  weights  in  its  place  until 
equilibrium  is  nearly  obtained.  Again  determine  the  zero  point 
and  calculate  from  the  sensibility  how  much  must  be  added  to 
or  subtracted  from  the  weights  on  the  right-hand  pan  in  order 
to  produce  equilibrium. 

Let  P  represent  the  unknown  weight  of  the  object,  p  the 
weights,  plus  or  minus,  which  must  be  added  to  P  to  produce 
equilibrium,  W  the  weight  on  the  right-hand  pan,  and  p1  the 
weight,  plus  or  minus,  which  must  be  added  to  W  in  order  to 
produce  equilibrium. 

Now,  since  P+p  and  W  +  p*  both  balance  the  load  on  the 
left-hand  pan,  they  are  equal  to  each  other,  and  P  =  W+p'  —p. 
The  proper  algebraical  sign  must  be  given  to  p  and  p'. 

This  procedure  is  also  called  the  method  of  weighing  by  sub- 
stitution. 

(b)  Place  upon  the  left-hand  pan  something  which  is  heavier 
than  the  object  to  be  weighed.  A  weight  or  a  small  beaker 
containing  shot  will  answer  the  purpose.  Put  the  thing  to  be 
weighed  upon  the  right-hand  pan  and  add  weights  until  equi- 
librium is  obtained.  Remove  the  object  and  add  more  weights 
until  equilibrium  is  again  obtained.  The  difference  between 
the  weights  required  in  the  two  cases  is  the  weight  of  the 
object. 

It  will  be  seen  that  the  principle  involved  in  this  method  is 
identical  with  that  on  which  Bunge's  balance  of  constant  sensi- 
bility is  constructed. 

III.  WEIGHING  BY  THE  METHOD  OF  GAUSS 

Weigh  the  object  as  directed  in  Exercise  II  under  II  (b)  first 
on  one  pan  and  then  on  the  other.  Multiply  the  two  weights 
together  and  extract  the  square  root  of  the  product. 


22  QUANTITATIVE  EXERCISES 

Let  W  represent  the  true  weight  which  is  to  be  found,  A 
the  weight  which  is  found  when  the  object  is  on  the  right-hand 
pan,  B  the  weight  obtained  when  it  is  on  the  left-hand  pan,  It  the 
length  of  the  right  arm,  and  L  the  length  of  the  left  arm. 

We  then  have 

WE  =  AL,  W*RL  =  ABEL, 

WL  =  BR,  W2  =  AB, 


Half  the  sum  of  A  and  B  may  be  taken  as  the  true  weight 
whenever  the  difference  between  —  -  —  and  V  'AB  is  too  small 
to  be  detected  by  the  balance. 


IV.  CORRECTION  FOR  THE  AIR  DISPLACED 

Just  as  objects  which  are  weighed  under  water  lose  weight 
by  an  amount  equal  to  the  weight  of  the  water  which  they  dis- 
place, so  those  weighed  in  the  air  are  lighter  than  they  would 
be  in  a  vacuum  by  the  weight  of  the  displaced  air. 

Suppose  two  ten-gram  brass  weights  which  have  been  found 
to  balance  each  other  in  a  vacuum  are  again  compared  in  the 
air.  They  will  be  found  to  balance  each  other  with  the  same 
precision  as  before ;  for,  having  the  same  specific  gravity,  their 
volumes  are  equal  and  they  must  therefore  lose  equally  by  the 
transfer.  Suppose,  on  the  other  hand,  that  one  of  the  two  ten- 
gram  weights  which  balance  each  other  in  a  vacuum  is  made  of 
brass. and  the  other  of  platinum.  If  these  are  compared  in  the 
air,  they  will  no  longer  be  found  to  be  equal ;  for,  having  differ- 
ent specific  gravities,  their  volumes  are  unequal,  and  they  must 
therefore  lose  unequally  by  the  transfer.  The  brass  weight, 
having  a  specific  gravity  of  8.4,  has  a  volume  of  1.19  cubic 
centimeters ;  while  the  platinum  weight,  with  a  specific  gravity 
of  21.5,  has  a  volume  of  only  0.465  cubic  centimeter.  The 
average  weight  of  a  cubic  centimeter  of  air  is  1.2  milligrams. 
The  brass  weight  will  therefore  lose  in  the  air  1.428  milligrams, 


THE  BALANCE  23 

while  the  platinum  weight  will  lose  only  0.558  milligram.  The 
former  will  appear  to  be  lighter  than  the  latter  by  the  difference 
between  1.428  and  0.558,  i.e.  by  0.87  milligram.  In  the  same 
way  it  may  be  shown  that  a  piece  of  quartz  weighing  ten  grams 
in  a  vacuum  would  appear  in  the  air  to  weigh  9.99887  grams  if 
weighed  with  brass  weights,  and  only  9.99602  grams  if  weighed 
with  platinum  weights ;  also  that  the  error  in  weighing  a  liter 
of  water  in  the  air  amounts  to  more  than  a  whole  gram  whether 
the  weights  are  of  brass  or  of  platinum. 

It  appears  that  weighings  conducted  in  the  air  are  correct 
only  when  the  object  weighed  and  the  weights  have  the  same 
specific  gravity,  and  that  when  the  two  —  the  object  weighed 
and  the  weights  —  differ  largely  in  respect  to  specific  gravity, 
the  errors  arising  from  the  buoying  effect  of  the  air  are  too  large 
to  be  neglected. 

To  find  the  true  weight  of  an  object  weighed  in  the  air,  it  is 
clear  that  we  must  add  to  or  subtract  from  its  apparent  weight 
the  difference  between  the  weights  of  the  air  displaced  by  the 
object  and  by  the  weights.  If  the  object  has  a  smaller  specific 
gravity  (i.e.  a  larger  volume)  than  the  weights,  this  difference 
must  be  added  ;  otherwise  it  must  be  subtracted. 

The  volumes  in  cubic  centimeters  of  the  object  weighed  and  of 
the  weights  —  and  consequently  of  the  air  displaced  by  each- 
are  found  by  dividing  their  apparent  weight  (weight  in  air)  in 
grams  by  their  respective  specific  gravities.  Strictly  speaking, 
the  method  is  slightly  erroneous  because  the  weights  found  in 
the  air  are  not  true  weights,  and  the  calculated  volumes  are 
therefore  not  quite  the  true  volumes*.  In  a  few  cases  this  error 
may  require  correction,  as,  for  example,  in  weighing  a  liter 
of  water,  in  which  instance  it  amounts  to  something  over  a 
milligram. 

Let  W  represent  the  true  weight  of  an  object  (i.e.  its  weight 
in  a  vacuum),  P  its  apparent  weight  (i.e.  its  weight  in  the  air), 
d  the  specific  gravity  of  the  object,  dl  the  specific  gravity  of 
the  weights,  and  a  the  weight  of  one  cubic  centimeter  of  air  at  the 


24  QUANTITATIVE  EXERCISES 

time  of  weighing.     Then  we  have  as  a  general  formula  for  the 
corrections  of  weighings  made  in  the  air 


d 

The  weight  of  a  cubic  centimeter  of  air  varies  with  the  pres- 
sure, the  temperature,  and  the  latitude,  also  with  the  moisture 
and  the  carbonic  acid  which  it  contains  ;  but  for  all  ordinary 
corrections  of  this  kind,  a  is  assumed  to  have  a  value  of  1.2 
milligrams,  and  the  equation  accordingly  becomes 

W=P  +  P  0.0012  A  -i- 
\d     d 

d1  =    8.4  when  the  weights  are  of  brass, 

21.55     "       "         "         "     "  platinum-iridium, 
21.50     "       "         "         "     "  platinum, 
2.65     "      "         "         "     "  quartz, 
2.60     "       u         "         "     "  aluminium. 

If,  as  happens  in  nearly  all  weighing  operations,  weights  of 
two  different  materials  are  used,  a  correction  for  each  must  be 
applied.  Suppose,  for  example,  it  is  desired  to  find  the  true 
weight  of  a  porcelain  vessel  (specific  gravity  2.29)  which  balances 
in  the  air  15  brass  grams  and  5  platinum  decigrams.  The  for- 
mula for  the  corrections  would  then  become 


=15.506  grams. 

A  table  will  be  found  in  Landolt-Boernstein's  PJiysikaliscJi- 

Chemische  Tabellen,  2.  Auflage,  page  10,  which  gives  the  com- 

puted values  of  0.0012  (-..  -  ~-J  between  d  =  .7  and  d  =  22  for 

\d     dj 

d1  =  8.4  (brass  weights),  d1  =  2.65  (quartz  weights),  and 
d1  =  21.55  (platinum  weights  containing  ten  per  cent  of 
iridium). 

The  first  chemist  to  adopt  the  practice  of  correcting  weigh- 
ings for  air  displacement  was  Edward  Turner  (1796-1837). 


THE  BALANCE  25 

Problem 

16.13161  grams  of  zinc  gave  20.08608  grams  of  zinc  oxide. 
14  grams  of  the  metal  and  15  grams  of  the  oxide  were  weighed 
with  brass  weights,  and  the  remainders  with  platinum.  The 
specific  gravity  of  zinc  is  7.15  and  that  of  the  oxide  5.65; 
what  were  the  true  weights  of  the  metal  and  of  the  oxide?  If 
the  atomic  weight  of  oxygen  is  16,  what  is  that  of  zinc? 

Weighing  by  Tares 

The  accuracy  of  weighings  may  be  considerably  increased 
by  employing  as  counterpoises  for  the  vessels  in  which  sub- 
stances are  weighed  (such  as  flasks,  crucibles,  tubes,  etc.)  other 
vessels  of  the  same  material,  form,  size,  and  weight.  The 
advantages  of  so  doing  are  :  (1)  the  vessels  and  their  tares  being 
of  the  same  material  and  presenting  the  same  area  of  surface  to 
the  air,  condense  or  absorb  the  same  amount  of  moisture,  and 
the  weighings  are  thus  freed  to  a  great  extent  from  the  errors 
which  arise  from  the  varying  humidity  of  the  atmosphere; 
(2)  having  the  same  density,  no  corrections  for  air  displacement 
are  necessary;  (3)  any  errors  resulting  from  changes  in  the 
weight  of  the  vessel  (due  to  heating,  immersion  in  liquids  or 
vapors,  or  to  other  necessary  treatment)  may  be  avoided  by 
subjecting  the  tare  to  precisely  the  same  treatment. 

In  practice,  the  lighter  of  the  two  vessels,  the  weights  of 
which  should  be  very  nearly  equal  to  begin  with,  is  selected 
as  the  tare ;  and  to  this  fragments  of  the  same  kind  of  material 
are  added  until  the  vessels  do  not  differ  in  weight  more  than  a 
fraction  of  a  milligram.  The  difference  in  their  weights  is  then 
carefully  determined  by  means  of  the  displacement  of  the  zero 
point  which  it  produces  and  the  sensibility  of  the  balance,  and 
allowance  is  made  for  it  in  all  subsequent  weighings. 


26  QUANTITATIVE  EXERCISES 

EXERCISE  III 
CORRECTION   OF  WEIGHTS 

I.    COMPARISON  OF  THE  DIFFERENT  PIECES  OF  A  SET  WITH 
EACH  OTHER 

Wherever  there  are  two  nominally  equal  weights  in  the 
set,  mark  one  of  them  by  making  a  slight  indentation  upon  it 
with  a  piece  of  steel  having  a  smooth  point.  Such  stamped 
weights  will  be  designated  in  what  follows  by  the  letter  s. 
Determine  the  zero  point  of  the  empty  balance.  Place  the  one- 
gram  weight  upon  one  pan  and  the  fractional  pieces  nominally 
equal  to  it  on  the  other.  Determine  the  zero  point  and  the 
sensibility  of  the  balance,  and  from  these  calculate  the  differ- 
ence between  the  two  loads.  Reverse  the  loads  and  again  find 
their  differences  in  weight  in  the  same  manner.  Find  their  true 
difference  in  weight  by  the  method  of  Gauss.  Regard  the  one- 
gram  piece  as  the  standard  weight  and  the  difference  between 
it  and  the  sum  of  the  fractional  pieces  as  the  error  of  the  latter. 
To  determine  the  value  of  the  larger  weights  as  compared  with 
the  standard,  make  the  following  weighings. 

2s  against  1  plus  the  fractional  weights,  assigning  to  the 
latter  their  true  value  as  compared  with  1, 

2  against  2s, 

5        «       2  +  28  +  1, 
10s       "       5  +  2  +  28  +  l, 
10        "       10s,  also  against  5  -f-  2  -f  2s  +  1, 
20        "       10  +  10s,  also  against  10s  +  5  +  2  +  2s  +  1, 
50        "       20  +  10  +  108  +  5  +  2  +  2s  +  1. 

It  remains  to  be  discovered  how  the  error  of  the  sum  of  the 
fractional  weights  is  distributed  among  the  several  pieces. 
Weigh  0.010s  against  0.00*5  +  0.002  +  0.002  +  0.001.  Regard 
0.010s  as  the  standard  fractional  weight,  and  the  difference 
between  it  and  the  smaller  pieces  as  the  error  of  the  latter. 


THE  BALANCE  27 

To  find  the  value  of  the  larger  fractional  pieces,  weigh : 

0.010  against  0.010s, 

0.020         «       0.010  +  0.010s, 

0.050        «       0.020  +  0.010  +  0.010s  +  0.005  +  0.002  +  0.002 

+  0.001, 
0.100s       «       0.050  +  0.020  +  0.010  +  0.010s  +  0.005  +  0.002 

+  0.002  +  0.001, 
0.100        «       0.100s, 
0.200        «       0.100  +  0.100% 
0.500        "       0.200  +  0.100  +  0.100s  +  0.050  +  0.020  +  0.010 

+  0.010  +  0.005  +  0.002  +  0.002  +  0.001. 

Having  thus  found  the  value  of  the  fractional  pieces  above 
0.005,  as  compared  with  0.010s,  bring  them  all  to  the  standard 
of  the  larger  weights,  the  one-gram  piece,  by  correcting  each 
for  its  share  of  the  total  error  (i.e.  the  difference  in  weight 
between  the  sum  of  the  fractional  pieces  and  the  standard  one- 
gram  weight).  In  making  this  correction,  0.005,  0.002,  0.002, 
and  0.001  will,  of  course,  be  regarded  as  a  single  weight.  If 
the  arms  of  the  balance  are  so  graduated  that  ten  full  milligrams 
can  be  added  to  them  by  means  of  the  rider,  as  in  the  Becker 
balance,  it  is  not  necessary-  to  carry  the  comparison  of  the 
weights  below  the  10-milligram  pieces.  If  the  arm  is  divided 
into  only  ten  equal  parts,  and  the  construction  of  the  beam  is 
such  that  the  rider  cannot  be  carried  quite  to  the  terminal  knife 
edges,  it  is  necessary  to  begin  the  work  upon  the  fractional 
weights  by  weighing  0.005  against  0.002  +  0.002  +  0.001.  The 
value  of  the  riders  is  found  by  weighing  them  against  a  corrected 
0.010-gram  piece.  If  only  one  arm  of  the  balance  is  graduated 
and  the  riders  weigh  twelve  milligrams  each,  it  will  be  necessary, 
in  order  to  obtain  weighings  on  both  sides,  to  weigh  the  riders 
against  0.010  +  0.002.  In  this  case  the  correction  of  the  weights 
must  be  extended  so  as  to  include  the  milligram  pieces. 

Having  found  the  values  of  the  various  weights  with  respect 
to  the  one-gram  piece,  any  other  weight  in  the  set  may  be  made 


28  QUANTITATIVE  EXERCISES 

the  standard  of  comparison.  Suppose  it  is  desired  to  find  the 
value  of  the  various  weights  with  respect  to  the  stamped  ten- 
gram  piece. 

Let  x  represent  the  value  of  any  weight  as  compared  with 
the  new  standard  (i.e.  the  value  to  be  found),  a  the  value  of 
the  same  weight  as  compared  with  the  one-gram  piece,  and 
b  the  value  of  10s  as  compared  with  the  one-gram  piece. 

The  equation  for  finding  the  value  of  x  will  then  be 

6:10:  iaix,  or  x=  10    - 


II.   To  COMPARE  A  SET  OF  WEIGHTS  WITH  A  STANDARD  WEIGHT 

Find  the  difference  between  the  standard  and  the  piece  of  the 
same  nominal  weight  in  the  set  by  the  method  of  Gauss.  Com- 
pare the  weights  with  each  other  and  find,  in  the  manner  pre- 
scribed under  I,  the  value  of  each  piece  with  respect  to  the 
standard  weight. 

As  a  rule,  the  balances  of  a  chemical  laboratory  are  so  unfa- 
vorably located  and  so  destitute  of  the  accessories  required  for 
refined  weighing,  that  it  is  difficult  to  make  satisfactory  com- 
parisons of  weights  with  them.  Fortunately,  however,  the  ad- 
justment of  weights  has  been  carried  by  several  makers  to  a 
very  high  degree  of  precision,  and  the  weights  which  are  fur- 
nished by  them  need  not  be  tested  unless  the  work  to  be  done 
demands  very  great  accuracy. 

Materials  for  Weights 

The  substances  commonly  employed  as  material  for  weights 
are  brass,  platinum,  platinum-indium,  quartz,  and  glass.  Brass 
is  inexpensive  and  is  easily  formed  and  adjusted,  but  is  too 
readily  corroded.  This  defect,  however,  is  often  remedied  by 
gold  plating  the  weights.  Platinum  is  noncorrodible  and  readily 
adjusted,  but  it  is  too  costly  and  has  a  specific  gravity  too  far 


THE  BALANCE  29 

above  that  of  a  majority  of  the  substances  commonly  weighed. 
Quartz  and  glass  are  not  easily  attacked  by  substances  likely  to 
find  their  way  into  the  air,  and  they  have  specific  gravities  quite 
near  to  those  of  many  of  the  substances  to  be  weighed ;  they  are, 
however,  in  consequence  of  their  greater  hardness,  more  difficult 
than  the  metals  to  fashion  and  adjust,  and  the  weights  made  from 
them  easily  become  electrified  when  they  are  removed  from  their 
places  in  the  box,  especially  when  the  box  is  lined  with  velvet 
or  leather. 


CHAPTER   II 


THE   BAROMETER   AND   THE   THERMOMETER 
THE   BAROMETER 

The  principal  use  of  the  barometer  in  a  chemical  laboratory  is 
in  connection  with  the  measurement  of  gases  and 
the  determination  of  the  boiling  points  of  liquids. 
The  instrument  usually  employed  is  either  the 
Fortin  modification  of  the  cistern  barometer  or 
the  so-called  siphon  barometer. 

The  simplest  form  of  the  cistern  barometer 
consists  of  a  wide  vessel,  the  cistern,  and  a  long 
glass  tube  closed  at  one  end  and  open  at  the 
other.  The  tube  is  filled  with  mercury,  inverted, 
and  its  open  end  immersed  in  mercury  which 
partly  fills  the  cistern.  The  pressure  of  the  air  is 
then  measured  by  the  height  to  which  the  mercury 
rises  in  the  tube,  i.e.  by  the  vertical  distance 
between  the  upper  surfaces  of  the  mercury  in  the 
cistern  and  in  the  tube.  The  principal  objection 
to  this  form  of  the  instrument  is  the  necessity  of 
employing  either  an  adjustable  scale,  or  —  if  fixed 
—  one  so  contrived  as  to  compensate  for  the 
difference  between  the  cistern  and  the  tube  in 
respect  to  cross  section.  In  the  Fbrtin  barometer, 
Fig.  1,  this  difficulty  is  obviated  by  employing  a 
cistern  having  a  flexible  leather  bottom  which  can 
be  raised  or  lowered  by  means  of  an  adjusting 
screw  placed  underneath.  By  this  means,  when- 
ever a  reading  is  to  be  taken,  the  level  of  the 
30 


FIG.  1 


THE  BAROMETER  AND  THE  THERMOMETER 


31 


mercury  in  the  cistern  is  made  to  coincide  with  the  lower  extrem- 
ity of  the  graduated  scale  upon  or  at  the  side  of  the  barometer 
tube.  The  correct  adjustment  of  the  surface  is  made  possible 
by  a  sharp-pointed  ivory  indicator,  which  is  fixed 
in  a  vertical  position  above  the  mercury  in  the 
cistern  in  such  a  manner  that  its  lower,  pointed 
end  lies  in  the  same  horizontal  plane  with  the  lower 
extremity  of  the  graduated  scale.  The  slightest 
contact  between  the  ivory  point  and  the  metal  pro- 
duces a  visible  depression  in  the  surface  of  the 
latter.  The  scale  may  be  etched  upon  the 
tube,  but  it  is  usually  of  brass  and  is  often 
provided  with  a  vernier,  Fig.  2,  and  with 
accessories  designed  to  obviate  those  errors 
in  reading  which  are  due  to  parallax. 

The  siphon  barometer,  Fig.  3,  is  so  called 
with  reference  to  its  form  only.  It  consists 
of  a  U-shaped  tube  having  one  limb  much 
longer  than  the  other.  The  long  limb  is 
closed  at  the  top  and  bent  so  as  to  bring  its 
upper  portion  into  line  with  the  shorter  limb. 
The  scale  is  usually  etched  upon  the  glass 
and  extends  over  both  limbs.  The  point 
from  which  the  enumeration  of  the  divisions 
of  the  scale  begins  may  be  above  the  high- 
est or  below  the  lowest  possible  level  of  the 
mercury  in  the  shorter  limb.  In  the  former  case 
the  numbers  increase  in  the  upward  direction  on 
the  longer  and  in  the  downward  direction  on  the 
shorter  limb,  and  the  readings  on  the  two  limbs 
are  to  be  added  ;  while  in  the  latter  case  the  num- 
bers increase  in  the  upward  direction  on  both  limbs  and  the 
readings  are  to  be  subtracted. 

There  are  certain  facts  concerning  the  barometer  and  its  use 
which  require  attention. 


FIG.  3 


FIG.  2 


32  QUANTITATIVE  EXERCISES 

1.  The  mercury  must  be  pure  in  order  that  its  density  may 
be  normal  and  its  movements  in  the  tube  free. 

2.  The  tube  of  a  cistern  barometer  must  be  of  sufficient  internal 
diameter  ;  since,  otherwise,  the  height  of  the  mercury  column  is 
sensibly  affected  by  capillary  depression.    In  the  siphon  barom- 
eter this  difficulty  disappears  except  in  so  far  as  the  meniscuses 
in  the  two  limbs  of  the  instrument  differ  in  form  owing  to  the 
fact  that  one  of  them  is  in  the  air  and  the  other  in  a  vacuum. 

3.  The  inclosed  space  in  the  tube  above  the  mercury  must 
be  free  from  air  and  moisture.     The  simple  filling  of  a  tube 
with  mercury  does  not  completely  remove  the  air.     A  small 
portion  of  it  clings  to  the  glass  and  this  can  be  expelled  only 
by  boiling  the  mercury  in  the  tube ;  hence  all  barometers  are 
"boiled  out"  at  the  time  of  filling.     Whether  the  inclosed 
space  is  free  from  air  or  not  may  be  ascertained  by  tilting  the 
instrument  to  one  side  until  the  mercury  rises  to  the  top  of  the 
tube,  when,  if  air  is  absent,  the   mercury  will  fill  the  entire 
space.     Another  way  of  testing  for.  the  presence  of  air  is  cau- 
tiously to  set  the  column  of  mercury  in  motion  until  it  strikes 
against  the  top  of  the  tube.     If  the  vacuum  is  complete,  a  sharp 
ringing  sound  will  be  produced  which  is  unmistakable. 

The  depression  of  the  barometer  which  is  produced  by  the 
vapor  of  the  mercury  itself  is  insignificant,  amounting  at  20° 
only  to  0.02  mm. 

4.  The  scale  by  which  the  height  of  the  barometer  is  read,  if 
not  known  to  be  correct,  must  be  tested.     This  may  be  done 
with  sufficient  accuracy  for  most  purposes  in  the   following 
manner.     A  pair  of  slender  sharp-pointed  dividers  with  screw 
adjustment  are  opened  on  any  part  of  the  graduation  until 
some  whole  number  of  divisions,  e.g.  ten,  are  included  between 
the  points.     Thus  adjusted,  the  dividers  are  run  several  times 
over  the  whole  scale,  beginning  each  time  at  a  new  place.     If 
the  points  are  always  found  to  span  the  same  number  of  divi- 
sions, the  graduation  is   sufficiently  uniform,   and  it  remains 
only  to  ascertain  whether  it  is  sufficiently  correct.     The  latter 


THE  BAROMETER  AND  THE  THERMOMETER     38 

question  is  quickly  settled  by  placing  the  dividers,  without 
alteration  of  adjustment,  upon  a  scale  known  to  be  correct, 
which  should  be  of  the  same  material  as  that  under  examination. 

5.  The  barometer,  when  read,  must  have  a  vertical  position, 
and  care  must  be  taken  to  avoid  errors  of  parallax.     If  practi- 
cable, a  telescope  should  be  employed. 

6.  The  siphon  barometer  should  be  gently  tapped  before  read- 
ing in  order  to  overcome  a  tendency  on  the  part  of  the  mercury  to 
lag  in  its  movements  in  the  tube.    This  treatment  is  not  necessary 
in  the  case  of  the  Fortin  barometer  when  the  column  has  been 
set  in  motion  by  the  adjustment  of  the  level  of  the  mercury  in 
the  cistern. 

CORRECTIONS  OF  THE  BAROMETER 

I.   For  Temperature 

Since  the  density  of  mercury  decreases  with  rising  tempera- 
ture, the  height  of  the  column  supported  by  a  given  atmospheric 
pressure  will  vary  somewhat  with  the  temperature  of  the  air 
(e.g.  the  pressure  which  supports  a  column  760  mm.  high  at  0° 
will,  at  20°,  support  one  762.76  mm.  in  length).  It  is  necessary, 
therefore,  to  select  some  standard  temperature  for  the  mercury 
and  to  correct  accordingly  all  readings  made  at  other  tempera- 
tures. The  standard  temperature  is  0°,  and  the  term  "  height 
of  the  barometer"  signifies  not  the  observed  height  but  that 
which  it  would  have  at  0°.  Since  the  height  of  the  column  of 
mercury  supported  by  the  weight  of  the  air  is,  in  general,  inde- 
pendent of  the  diameter  of  the  tube,  the  corrected  height  (b) 
will  be  found  by  dividing  the  observed  height  (b!)  by  unity  plus 
the  cubical  expansion  of  mercury  (0.0001818  t)  from  0°  to  the 
temperature  at  which  the  reading  was  made.  The  general 

formula  for  the  correction  is  b  =  z .,    ^      »  when  the  tem- 

1  -\-  O.UUUlolo  t 

perature  is  above  0°;  and  b  =  ^ — A  AAAI  QI  Q  S  wnen  the  tem- 

1  —  U.UUUlolo  t 

perature  is  below  0°. 


84  QUANTITATIVE  EXERCISES 

The  effect  of  change  in  temperature  upon  the  length  of  the 
scale  must  also  be  taken  into  account,  for  its  graduation  is  correct 
only  at  the  standard  temperature  of  0°.  Obviously  the  divisions 
are  too  far  apart  at  temperatures  above  0°,  and  too  near  together 
at  temperatures  below  that  point.  If  b  and  t  have  the  same 
significance  as  in  the  previous  formulas,  while  7  represents  the 
linear-expansion  coefficient  of  the  material  of  the  scale,  the 
formula  for  the  correction  of  the  scale  is  Z  =  6(1  4-7*),  or 
1  =  6(1  —  7*),  according  as  the  temperature  is  above  or  below  0° 
at  the  time  of  reading. 

Letting  h  represent  the  true  height  of  the  barometer,  we  have, 
by  combining  the  two  corrections,  the  formulas 


r        j, 


0.0001818* 
for  temperatures  above  0°,  and 

,      ,,          I-* 
0  1-0.0001818* 

for  temperatures  below  0°. 

7  =  0.0000085  when  the  scale  is  of  glass,  and  0.0000184  when 
it  is  of  brass. 

It  will  be  observed  that  the  two  errors  —  the  one  due  to 
change  in  the  density  of  the  mercury  and  the  one  due  to  change 
in  the  length  of  the  scale  —  tend  to  compensate  each  other  at 
all  temperatures. 

Simpler  formulas  for  the  temperature  correction  of  the  barom- 
eter which,  though  not  entirely  accurate,  are  sufficiently  so  for 
most  purposes,  are  : 

h  =  b'(l-  (0.0001818  -  7)  t)  for  temperatures  above  0°, 
and  h  =  b'(I  +  (0.0001818  -  7)  t)  for  temperatures  below  0°. 
These  become 

h  =  b'  (1-0.0001733*)  and 

h  =  bf  (1  +  0.0001733  t)  when  the  scale  is  of  glass, 
and   h  =  fc'(l-  0.0001634  «)  and 

h  =  b'  (1  +  0.0001634  t)  when  the  scale  is  of  brass. 


THE  BAROMETER  AND  THE  THERMOMETER     35 

II.  For  Capillary  Depression 

As  previously  stated,  there  is  no  correction  to  be  made  for 
capillary  depression  in  the  siphon  barometer.  The  depression 
is  likewise  negligible  in  cistern  barometers  when  the  diameter 
of  the  mercury  column  is  equal  to  or  exceeds  25  mm.  In 
narrower  tubes,  however,  the  depression  sensibly  affects  the 
height  of  the  barometer,  and  is  to  be  added  to  the  readings  of 
the  instrument  unless  the  correction  has  already  been  provided 
for  in  the  scale  itself. 

The  following  table  gives  the  amount  of  the  correction  for 
tubes  of  various  small  diameters. 

DIAMETER  OF  TUBE  DEPRESSION  DIAMETER  or  TUBE  DEPRESSION 

4  mm 1.64  mm.  14  mm 0.12  mm. 

G    « 0.91    «  16    « 0.07    « 

8    « 0.54    «  18    « 0.04    « 

10    " 0.32    «  20    " 0.03    « 

12    « 0.19    « 

III.  For  Latitude  and  Altitude 

The  atmospheric  pressure  which  will  support  a  column  of 
mercury  of  given  height,  e.g.  760  mm.,  is  not  a  fixed  quantity, 
but  diminishes  as  the  distance  from  the  earth's  center  of  gravity 
increases.  It  varies,  therefore,  with  latitude  and  elevation  above 
sea  level.  Hence  it  is  necessary,  even  for  meteorological  com- 
parisons, to  establish  a  unit  for  the  earth's  attraction  and  to 
reduce  observations  on  the  height  of  the  barometer  in  all  parts 
of  the  earth's  surface  to  this  standard.  The  unit  agreed  upon 
is  the  force  of  gravity  which  is  found  to  prevail  at  sea  level  in 
latitude  45°.  If  it  is  desired  to  reduce  a  reading  of  the  barom- 
eter made  at  any  other  latitude  and  altitude  to  the  height 
which  it  would  have  —  with  the  same  atmospheric  tension  —  at 
latitude  45°  and  sea  level,  the  observed  height,  corrected  for 
temperature,  is  to  be  multiplied  by  the  expression 
(1  -  0.0026  cos  2c£)  (1  -  0.0000002  H), 


36  QUANTITATIVE  EXERCISES 

in  which  $  is  the  latitude  of  the  place  and  If  its  elevation  in 
meters  above  sea  level.  The  latter  term,  (1  -  0.0000002  H), 
may  be  neglected  except  when  the  altitude  of  the  place  is  con- 
siderable, since  at  an  elevation  of  500  meters  the  correction  for 
altitude  amounts  to  only  0.07  mm. 

The  following  table  will  give  an  idea  of  the  magnitude  of 
the  corrections  for  latitude  when  the  observed  height  of  the 
barometer  is  760  mm. 

Latitude         =       0°        12°        18°     '  24°        30°        36°       42°    45° 
Correction  ^  =  1.97      1.80      1.59      1.31      0.98      0.61      0.21       0.0  mm. 
Latitude          =     90°        78°        72°        66°        60°        54°        48°     45° 

The  correction  is  subtracted  from  the  height  of  the  barometer, 
or  added  to  it,  according  as  the  latitude  of  the  place  of  obser- 
vation is  below  or  above  45°.  It  will  be  observed  that,  with 
equal  atmospheric  pressure  at  the  two  places,  a  barometer  at 
the  equator  would  stand  3.94  mm.  higher  than  one  at  the  pole. 

What  has  been  said  of  the  mutual  relations  of  barometric 
height,  atmospheric  pressure,  and  the  force  of  gravity  is  also  true 
of  every  other  column  of  mercury  which  is  supported  by  the  pres- 
sure of  a  gas  or  by  the  difference  between  two  gas  pressures. 
The  bearing  of  this  fact  upon  the  work  of  the  chemist  will  be 
made  clearer  by  outlining  the  most  common  method  of  measur- 
ing a  gas.  A  tube,  closed  at  one  end  and  graduated,  is  filled 
with  mercury,  inverted,  and  its  open  end  immersed  in  mercury 
in  an  open  vessel.  The  apparatus  thus  prepared  resembles  a 
cistern  barometer,  and  is  one,  in  fact,  if  the  length  of  the  tube 
is  equal  to  or  greater  than  a  barometric  height.  The  gas  tq  be 
measured,  on  being  introduced  into  the  tube  at  the  open  end, 
rises  to  the  top  and  depresses  the  column  of  mercury.  Its 
volume  is  then  read  upon  the  graduation  of  the  tube.  But  the 
term  volume  has  no  significance  in  the  case  of  a  eras  unless 

o  o 

the  pressure  under  which  it  is  measured  is  known,  because 
its  volume,  leaving  temperature  out  of  the  consideration, 
depends  on  the  pressure.  Hence  it  is  necessary  to  ascertain 


THE  BAROMETER  AND  THE  THERMOMETER     37 

the  pressure  resting  upon  a  gas  at  the  time  of  determining  its 
volume.  This  is  a  very  simple  matter ;  for,  evidently,  the  pres- 
sure upon  the  gas  which  determines  the  space  it  can  occupy  is 
the  difference  between  the  pressure  of  the  air  and  the  pressure 
of  the  column  of  mercury  in  the  eudiometer,  and  this  will  be 
found  by  subtracting  the  height  of  the  column  of  mercury  in 
the  tube  from  the  height  of  the  barometer.  Stated  in  another 
way,  the  sum  of  the  pressure  exerted  by  the  gas  and  that  exerted 
by  the  column  of  mercury  in  the  eudiometer  is  equal  to  the  pres- 
sure of  the  atmosphere ;  and  hence,  to  find  the  pressure  exerted 
by  the  gas,  we  have  only  to  subtract  the  height  of  the  one  mer- 
cury column  from  that  of  the  other.  The  result  is  the  height 
of  the  mercury  column  which  the  gas  is  capable  of  supporting, 
and  it  is  the  measure  of  its  pressure  or  tension  in  the  same  sense 
that  the  height  of  the  barometer  is  the  measure  of  the  pressure 
or  tension  of  the  atmosphere. 

Having  thus  found  the  volume  of  a  gas  when  measured  under 
some  known  pressure,  we  can  calculate  what  space  it  would 
occupy  under  any  other  pressure,  —  for  instance,  under  the 
conventional  standard  pressure,  which  is  the  pressure  required 
to  support  at  0°  a  vertical  column  of  mercury  760  mm.  in 
length.  Having  found  its  volume  under  standard  pressure,  we 
can  find  its  weight  through  the  known  density  of  the  gas. 

This  procedure  for  finding  the  weight  of  gases  suffices  for  all 
ordinary  purposes ;  but  when  great  accuracy  is  required,  as  in 
the  determination  of  an  atomic  weight,  it  is  necessary  to  take 
into  account  the  fact  already  explained  that  the  height  of  a  col- 
umn of  mercury  is  not  an  unvarying  measure  of  the  pressure 
which  supports  it.  In  other  words,  it  is  necessary  to  multiply 
the  height  of  the  mercury  column  which  expresses  the  pressure 
under  which  the  gas  was  measured  by  (1  —  0.0026  cos  2$) 
(1  -  0.0000002  H)  in  order  to  find  what  its  height  would  be  in 
the  standard  locality,  i.e.  latitude  45°  and  sea  level.  If  this 
correction  is  neglected,  equal  volumes  of  the  same  gas  will  be 
found  to  have  different  weights  in  different  parts  of  the  earth. 


38  QUANTITATIVE  EXERCISES 

IV.   For  the  Moisture  in  the  Air 

It  is  sometimes  desired  to  know  what  portion  of  the  observed 
height  of  the  barometer  or  of  another  mercury  column  is  due  to 
air  or  gas  in  the  dry  condition.  This  is  found  by  subtracting 
from  the  observed  height  of  the  column  the  tension  of  the 
water  vapor  expressed  in  millimeters  of  mercury.  If  the  gas  is 
saturated,  the  tension  of  the  water  vapor  will  be  known.  Other- 
wise it  must  be  determined. 


THE   THERMOMETER 

Three  things  are  to  be  ascertained  with  respect  to  a  ther- 
mometer before  it  can  be  used  with  confidence.  They  are  :  (1)  the 
position  of  the  upper  end  of  the  mercury  column  when  pure 
water  freezes, — its  zero  point;  (2)  the  position  of  the  same  when 
pure  water  boils,  —  its  boiling  point;  and  (3)  the  inequalities  of 
its  bore. 

An  instrument  having  the  scale  upon  the  glass  and  without 
the  white  opaque  background  so  common  in  laboratory  ther- 
mometers is  to  be  preferred  for  the  exercises  which  follow. 


EXERCISE  IV 

DETERMINATION   OF   THE   ZERO   POINT 

There  are  required : 

1.  A  strong  test  tube,  25  X  175  mm. 

2.  A  stout  platinum  wire,  250  or  275  mm.  long. 

3.  Two  corks,  one  of  which  fits  the  test  tube,  and  the  other 
large  enough  to  bore  for  the  tube. 

4.  A  beaker,  100  x  175  mm.,  or,  better,  a  glass  battery  jar  of 
approximately  the  same  dimensions. 

5.  A  cover  of  wood  or  metal  for  the  jar. 

6.  A  glass  tube  or  copper  wire,  8  x  600  mm. 


THE  BAROMETER  AND  THE  THERMOMETER     39 

Bore  the  smaller  cork  centrally  for  the  thermometer,  and  near 
the  outer  edge  for  the  platinum  wire ;  and  the  larger  cork  for 
the  test  tube. 

Bend  one  end  of  the  platinum  wire  into  a  ring  which  fits 
loosely  the  inside  of  the  tube  (1),  and  the  rest  of  the  wire  into 
a  right  angle  with  the  ring.  This  stirrer  may  also  be  made 
from  a  thin  glass  rod  or  tube  ;  or  a  ring  of  platinum  wire  with 
a  short  overlap  of  wire  bent  to  a  right  angle  may  be  made,  and 
the  standing  end  of  the  wire  fused  into  the  end  of  a 
glass  tube. 

Make  a  similar  stirrer  for  the  jar  or  beaker  (4)  out 
of  the  glass  tube  or  copper  wire. 

Bore  the  cover  (5)  for  the  cork  on  the  test  tube 
and  for  the  stirrer. 

The  apparatus  ready  for  use  is  shown  in  Fig.  4. 

Partly  fill  the  test  tube  with  distilled  water  and 
insert  the  thermometer.  The  distance  between  the 
lower  extremity  of  the  bulb  and  the  bottom  of  the  test 
tube  should  be  somewhat  greater  than  that  between 
the  bulb  and  the  side  of  the  tube,  and  the  level 
of  the  water  should  be  above  the  zero  point  of  the  FIG  4 
thermometer.  Move  the  stirrer  up  and  down  and 
make  sure  that  it  does  not  rub  against  the  bulb  at  any  point. 

Fill  the  outer  vessel  with  a  mixture  of  pulverized  ice,  or 
snow,  and  a  little  salt;  and  if  the  ice  is  quite  dry,  add  a  little 
water.  Place  the  tube  containing  the  thermometer  and  the 
water  in  position  in  the  larger  vessel,  and  stir  from  time  to 
time  the  freezing  mixture.  The  temperature  of  the  water  in 
the  inner  vessel  will  fall  somewhat  below  the  freezing  point 
before  the  formation  of  ice  begins.  As  soon  as  the  thermometer 
indicates  an  under-cooled  condition,  stir  the  water  vigorously 
in  order  to  prevent  the  formation  of  coarse  ice  and  to  secure  a 
uniform  distribution  of  temperature.  The  mercury  will  fise 
slightly  and  become  stationary,  giving  the  point  to  be  recorded. 
Remove  the  test  tube  from  the  freezing  mixture  and  resume  the 


40  QUANTITATIVE  EXERCISES 

stirring.  The  thermometer  will  remain  unchanged  until  a  large 
portion  of  the  ice  has  melted.  When  so  much  of  the  ice  has 
disappeared  that  the  thermometer  again  begins  to  rise,  return 
the  tube  to  the  freezing  mixture  and  repeat  the  experiment. 

Immerse  the  thermometer  bulb  in  the  steam  from  boiling 
water  for  half  an  hour  and  then  redetermine  the  zero  point. 

It  will  be  found  that  the  second  zero  point  is  lower  than  the 
first.  Repeat  the  heating  in  steam  and  again  determine  the 
zero  point.  The  second  and  third  results  should  agree. 

It  is  of  some  advantage  to  "jacket"  the  .apparatus  with  a 
larger  tube,  and  thus  provide  an  air  space  between  the  cooling 
mixture  and  the  water.  The  freezing  of  the  water  and  the 
melting  of  the  ice  are  then  slower,  giving  more  time  for  the 
observation  of  the  thermometer. 

A  very  common  but  less  satisfactory  procedure  for  the  deter- 
mination of  the  zero  points  of  thermometers  will  be  described. 
A  piece  of  rubber  tubing  provided  with  a  pinchcock  is  attached 
to  the  stem  of  a  large  funnel,  which  is  placed  in  the  ring  of 
an  iron  stand  and  filled  with  snow  or  finely  powdered  ice.  The 
ice,  if  dry,  is  then  moistened  with  enough  distilled  water  to 
expel  the  air.  The  bulb  of  the  thermometer,  thinly  wrapped 
in  flannel  to  prevent  actual  contact  with  the  icej  is  imbedded 
in  the  ice  to  a  point  on  the  stem  above  the  zero  mark.  From 
time  to  time  the  thermometer  is  raised,  or  the  funnel  lowered, 
in  order  to  observe  the  height  of  the  mercury. 

There  are  two  objections  to  this  method:  (1)  the  tempera- 
ture of  the  ice  may  be  below  the  freezing  point  of  water,  in 
which  case  the  water  in  contact  with  the  bulb,  despite  the 
flannel  covering,  may  be  under-cooled ;  (2)  the  ice  may  con- 
tain soluble  impurities,  which,  even  in  small  quantities,  sensibly 
depress  the  melting  point. 

The  latter  difficulty  may  be  avoided  in  the  following  man- 
ner. A  metallic  rod  of  the  same  diameter  as  the  thermometer 
is  fixed  in  a  vertical  position,  its  lower  end  immersed  in  a  vessel 
filled  with  distilled  water,  which  is  afterwards  frozen  to  a  solid 


THE  BAROMETER  AND   THE  THERMOMETER  41 

cake  pf  ice  by  surrounding  it  with  a  freezing  mixture.  The 
rod  is  loosened  by  heating  its  free  end,  and  removed,  leaving  a 
cavity  for  the  bulb  of  the  thermometer.  The  stem  of  the  ther- 
mometer is  provided  with  a  rubber  ring  wider  than  the  hole, 
which  rests  on  the  ice  when  the  instrument  is  in  position. 


The  difference  which  was  observed  in  the  foregoing  experi- 
ment between  the  zero  points  determined  before  and  after  heat- 
ing the  thermometer  to  the  boiling  point  of  water  is  due  to  the 
fact  that  when  glass  is  heated  and  again  cooled  to  its  former 
temperature  it  .does  not  immediately  shrink  to  its  original  vol- 
ume. In  other  words,  the  depression  of  the  zero  point  which 
follows  a  heating  of  the  thermometer  is  the  result  of  an  enlarge- 
ment of  the  bulb. 

The  extent  of  the  depression  varies  greatly  with  the  composi- 
tion of  the  glass,  being  large  in  the  Thuringian,  and  small  in 
the  so-called  Jena  normal  glass,  which  contains  in  addition  to 
the  more  common  constituents  of  glass  —  silica,  alkalies,  and 
lime  —  about  7  per  cent  of  zinc  oxide  and  2  per  cent  of  boric 
acid.  It  also  varies  with  the  temperature  to  which  the  ther- 
mometer is  heated  and  with  the  duration  of  the  heating. 

The  dilatation  is  not  permanent.  The  shrinkage  of  the  bulb 
begins  at  once ;  for  if  a  thermometer  which  has  been  heated  to 
the  boiling  point  of  water  is  placed  in  melting  ice,  the  mercury 
is  observed  to  sink  to  a  certain  point,  and  then  slowly  to  rise. 
This  lowest  reading  is  known  as  the  "  zero  of  maximum  depres- 
sion." The  rate  at  which  the  bulb  returns  to  its  original  vol- 
ume, like  the  degree  of  the  dilatation,  is  variable.  The  recovery 
may  last  a  few  hours  or  several  months  according  to  the  com- 
position of  the  glass,  the  temperature  to  which  the  thermometer 
has  been  heated,  the  duration  of  the  heating,  and  the  rapidity 
of  the  cooling.  In  the  case  of  new  thermometers,  a  gradual 
slow  rise  of  the  zero  point  is  observed  which  may  require  years 
for  its  completion. 


42  QUANT1TATJV1.    K\1.K<  [SE8 

If  a  thermometer  is  heated  for  a  long  time  to  a  hi^li  tem- 
perature, the  zero  point  rises,  and  in  some  cases  to  an 
ishing  extent     Crafts,  after  heating  certain  thermometers  of 
common  German  and  of  French  crystal  glass  to  355°  for  < . 
days,  observed  elevations  of  the  zero  points  ranging  from  11 
to  26°.     At  the  same  time  the  intervals  between  the  free/in^- 
and  boiling  points  had  increased  by  quantities  ranging  from 
0.45°  to  0.9°,  showing  that  the  expansion  coefficient  of  the 
glass  had  been  diminished  by  the  prolonged  heating. 

The  lack  of  constancy  in  the  position  of  the  zero  point  — 
from  which  temperature  is  reckoned  —  does  not  seriously  inter- 
fere with  the  accuracy  of  the  thermometer  as  an  instrument  for 
the  measurement  of  temperature  since,  in  practice,  one  proceeds 
in  such  a  manner  as  to  eliminate  errors  from  this  source.  When 
a  determination  is  to  be  made  with  a  thermometer,  the  instru- 
ment is  first  heated  for  some  time  to  the  temperature  which  it 
is  afterwards  to  register,  and  its  zero  point  then  ascertained. 
Or,  what  is  better,  if  the  heating  is  a  prolonged  one,  the  zero 
point  of  the  thermometer  is  determined  immediately  after  the 
completion  of  the  experiment. 

A  thin  column  of  mercury  moving  through  a  glass  tube  is 
considerably  retarded  by  capillary  resistance ;  consequently,  at 
the  same  temperature,  somewhat  different  readings  mav  l>e 
obtained  according  as  the  thermometer  is  observed  after  a  rise 
or  after  a  fall  of  the  mercury.  This  effect,  known  as  "stiction," 
is  especially  troublesome  in  thermometers  with  very  fine  threads, 
i  be  overcome  to  a  great  extent  by  jarring  the  instrument; 
hence  it  is  customary  before  reading  a  thermometer  to  tap  it 
sharply  with  one  of  the  fingers,  with  a  piece  of  hard  wood,  or 
—  more  lightly  —  with  a  piece  of  metal.  A  very  satisfactory 
device  for  this  purpose  is  a  minute  electrical  hammer  constructed 
like  a  vibrating  bell,  which  is  attached  to  the  upper  end  of  the 
thermometer  and  kept  in  operation  while  the  mercury  is  moving 
up  or  down  to  its  position  of  rest 


TILE   IJAIIOMETEK   AND  THE  THERMOMETER  43 


~ 


DETERMINATION  OF  THE  BOILING  POINT 

A  metallic  vessel  constructed  on  the  plan  of  that  represented 
in  Fig.  5  is  required.  One  of  the  small  horizontal  tubes  serves 
for  the  escape  of  the  steam.  To  the  other  is  attached  a  glass 
U-tube  partly  filled  with  water  to  indicate  —  and  to  measure  if 
necessary — any  difference  in  pressure  between  the  steam  within 
the  vessel  and  the  external  atmosphere.  The  bulb  must  be 
above  the  water  in  the  vessel  and  the  column  of  mercury  should 
reach  only  a  very  short  distance  above  the  cork 
which  holds  the  thermometer  in  place.  In  pre- 
paring the  cork  provision  is  to  be  made  for  the 
pliiH||jT  of  steam  through  it  along  the  stem  of 
the  thermometer. 

Boil  the  water  rapidly  for  ten  or  fifteen  min- 
utes ;  then  note  the  height  of  the  thermometer 
and  any  difference  in  the  level  of  the  water  in  the 
two  limbs  of  the  manometer.  Record  also  the 
height  and  the  temperature  of  the  barometer. 

The  temperature  at  which  water  boils  depends, 
of  course,  upon  the  pressure  of  the  air.  The  next  step,  therefore, 
is  to  correct  the  observed  height  of  the  barometer  to  the  height 
which  it  would  have  at  the  standard  temperature  of  0°,  and  to 
correct  the  true  height  of  the  barometer,  in  turn,  for  any  differ- 
ence of  pressure  indicated  by  the  manometer.  The  reduction  of 
the  barometer  reading  has  already  been  explained.  The  amount 
of  fhe  second  correction  is  found  in  the  following  manner.  The 
temperature  of  the  water  in  the  manometer  is  ascertained  and 
the  density  of  water  at  that  temperature  is  found  in  the  appro- 
priate tables.  The  product  of  the  difference  in  the  level  of  the 
water  in  the  two  limbs  and  the  density  of  the  water,  as  found 
in  the  appropriate  table,  is  then  divided  by  the  density  of  mer- 
cury at  0°,  Le.  by  13.5956. 


44 


QUANTITATIVE  EXERCISES 


The  pressure  found  could,  of  course,  be  further  reduced  to 
its  value  at  latitude  45°  and  sea  level ;  but  this  last  refinement 
would  hardly  be  justified  in  the  case  of  thermometers  which  can- 
not be  accurately  read  to  within  less  than  0.05°,  since  the  effect 
of  a  correction  for  latitude  upon  the  boiling  point  of  water 
amounts  —  with  a  pressure  of  760  mm.  of  mercury  —  only  to 
0.037°  at  latitude  30,  0.025°  at  35,  and  0.013°  at  40.  The 
correction  for  any  moderate  altitudes  is  much  smaller,  amount- 
ing at  an  elevation  of  500  meters  only  to  0.0026°. 

Having  found  the  pressure  under  which  the  water  was  boiled, 
the  temperature  corresponding  to  that  pressure,  i.e.  the  true 
boiling  point,  may  be  obtained  from  the  tables.  The  following 
table  gives  the  boiling  points  of  water  at  pressures  between  740 
and  780  mm.  of  mercury. 


Boiling  Points  of  Water  at  Latitude  4$°  and  Sea  Level 


740 

99.26° 

750 

....  99.63° 

760  .  . 

.  .  100.00° 

770 

100.37° 

741 

99.30 

751 

.  .  .  .  99.67 

761  .  . 

.  .  100.04 

771 

.  .  .  .  100.40 

742 

99.33 

752 

99.71 

762  .  . 

.  .  100.07 

772 

.  .  .  .  100.44 

743 

99.37 

753 

99.74 

763  .  . 

.  .  100.11 

773 

100.47 

744 

99.41 

754 

99.78 

764  .  . 

.  .  100.15 

774 

.  .  .  .  100.51 

745 

.  .  .  .  99.45 

755 

99.82 

765  .  . 

..  .  100.18 

775 

....  100.55 

746 

.  .  .  .  99.48 

756 

99.85 

766  .  . 

.  .  100.22 

776 

100.58 

747 

.  .  .  .  99.52 

757 

99.89 

767  .  . 

.  .  100.26 

777 

100.62 

748 

.  .  .  .  99.56 

758 

99.93 

768  .  . 

.  .  100.29 

778 

100.65 

749 

99.59 

759 

99.96 

769  .  . 

.  .  100.33 

779 

100.69 

780 

.  100.73 

Within  all  ordinary  ranges  of  the  barometer  a  change   of 
1  mm.  in  its  height  affects  the  boiling  point  of  water  0.0375°. 


THE  BAROMETER  AND  THE  THERMOMETER  45 

EXERCISE  VI 
CALIBRATION   OF   THE   THERMOMETER 

There  are  required  : 

1.  A  strip  of  good  mirror  glass  if  the  thermometer  is  a  trans- 
parent one.     Otherwise  it  cannot  be  used. 

2.  A  strong  magnifying  glass,  or,  better  still,  a  small  reading 
telescope  which  is  mounted  on,  and  movable  along,  a  stationary 
horizontal  rod. 

The  first  step  to  be  taken  is  the  separation  of  a  thread  of 
mercury  of  suitable  length.  If  the  thermometer  to  be  cali- 
brated is  one  of  wide  range,  e.g.  from  a  few  degrees  below  zero 
to  360°,  as  will  probably  be  the  case,  a  thread  spanning  from 
30  to  50  divisions  of  the  scale  will  suffice. 

Invert  the  thermometer  and  give  it  a  smart  tap  against  the 
table  top  or  the  palm  of  the  hand.  If  a  thread  separates,  it  will 
probably  be  too  long  or  too  short.  If  too  long,  return  the  thread 
nearly  to  the  bottom  of  the  scale  and  note  the  position  of  its 
lower  end.  Warm  the  bulb  until  the  mercury  in  it  rises  and 
joins  the  thread  in  the  stem,  removing  the  thermometer  from 
the  source  of  heat  the  moment  the  junction  is  established.  The 
cause  of  the  detachment  of  the  thread  is  a  minute  bubble  of 
air  which  is  usually  located  at  the  point  where  the  stem  joins 
the  bulb.  If  the  bulb  is  subsequently  heated,  the  air  bubble  is 
pushed  along  until  the  rising  mercury  joins  the  thread,  when 
it  lodges  on  the  side  of  the  tube  and  remains  attached  to  the 
glass  in  its  new  position.  The  next  break  in  the  thread  will 
occur  at  this  point.  Now  cool  the  bulb  until  a  thread  of  the 
right  length  remains  above  the  place  of  junction ;  then  invert 
and  tap  as  before.  A  thread  of  the  desired  length  should  sepa- 
rate. If  it  does  not,  the  operation  is  to  be  repeated  until  it  is 
successful. 

If,  on  the  other  hand,  the  thread  first  detached  is  too  short, 
proceed  as  before  and  continue  to  warm  the  bulb  until  a  column 


46  QUANTITATIVE  EXERCISES 

of  mercury  of  the  right  length  has  risen  above  the  place  of  junc- 
tion and  then  effect  the  separation  in  the  usual  manner. 

When  a  thermometer  is  inverted  and  struck  against  a  hard 
object  in  the  manner  described,  it  frequently  happens  that  the 
air  bubble  which  is  expected  to  cause  the  detachment  of  a  thread 
at  the  junction  of  the  stem  and  the  bulb  glides  along  the  wall 
of  the  bulb  towards  its  highest  part,  while  the  mercury  flows 
out  of  the  bulb  and  fills  the  entire  stem.  In  such  cases  the 
bubble  is  to  be  brought  back  to  the  junction  of  the  stem  and 
the  bulb.  This  can  be  done  by  returning  the  thermometer  with 
a  quick  movement  to  the  upright  position  and  by  tapping  the 
-bulb  simultaneously  against  the  table.  The  thread  may  then  be 
broken  off,  and  shortened  or  lengthened  in  the  manner  described. 

The  detached  thread  may  be  brought  with  great  exactness  to 
any  desired  position  under  the  graduation  by  inclining  the  ther- 
mometer and  tapping  against  the  ends,  but  skill  in  placing  the 
thread  can  be  acquired  only  by  practice. 

The  correct  determination  of  the  position  under  the  scale  of 
the  ends  of  the  mercury  column  is  also  a  matter  of  some  dif- 
ficulty at  first,  owing  to  errors  of  parallax  and  to  inability  to 
estimate  correctly  fractions  of  a  division  of  the  graduation.  The 
latter  difficulty  disappears  with  practice,  but  errors  of  parallax 
can  be  avoided  only  by  the  employment  of  suitable  means.  The 
best  of  these  is  the  reading  telescope  mounted  in  the  manner 
previously  referred  to.  If  this  instrument  is  not  to  be  had,  the 
scale  may  be  observed  through  a  powerful  pocket  lens  which  is 
fixed  in  a  ring  supported  by  three  legs.  Under  such  a  glass,  a 
line  Avhich  crosses  the  optical  axis  will  appear  straight,  while 
lines  on  either  side  of  it  will  appear  to  be  curved.  A  weak  lens 
is  of  little  assistance  owing  to  the  slightness.of  the  curvature 
which  it  gives  to  lines  not  crossing  its  axis.  Finally,  if  the 
thermometer  is  a  transparent  one,  it  may  be  mounted  on  blocks 
over  a  strip  of  mirror  glass.  The  proper  position  -for  reading 
will  then  be  found  by  bringing  the  eye,  the  end  of  the  thread, 
and  its  image  into  line. 


THE  BAROMETER  AND  THE  THERMOMETER     47 

Having  made  the  best  practicable  arrangements  for  the  avoid- 
ance of  parallax  and  for  magnifying  the  scale,  and  having 
detached  a  thread  of  mercury  of  the  desired  length,  run  the 
thread  nearly  down  to  the  bulb  and  place  its  lower  end  exactly 
opposite  the  scale  division  at  which  it  is  proposed  to  begin  the 
calibration.  Record  the  position  of  both  ends,  and  then  run 
the  thread  up  the  stem  until  its  lower  end  exactly  coincides 
with  the  previous  position  of  its  upper  end.  Record  the  new 
position  of  the  upper  end.  Proceed  hi  this  way  until  the 
thread  has  passed  through  as  much  of  the  stem  as  it  is  desired 
to  calibrate,  and  then  reverse  the  operation,  running  the  thread 
down  the  stem  and  recording  the  succeeding  positions  of  its  lower 
end.  The  readings  while  returning  should  not  be  compared 
with  those  recorded  while  going  up  until  the  work  is  completed. 

On  comparing  the  two  records,  the  readings  at  corresponding 
points  should  be  found  to  agree  very  closely ;  though  they  need 
not  and  probably  will  not  be  identical  if  one  attempts  to  esti- 
mate very  small  fractions  of  a  scale  division.  If  the  discord- 
ance is  considerable,  it  is  probably  due  to  the  inexperience  of 
the  experimenter  and  the  work  should  be  repeated  until  a  satis- 
factory agreement  between  the  upward  and  the  downward  read- 
ings is  obtained.  When  readings  at  corresponding  points  are 
found  to  disagree  slightly,  the  mean  of  all  is  to  be  accepted  as 
probably  safer  than  any  single,  observation. 

The  errors  in  reading  will  tend  to  accumulate  in  a  single 
direction ;  but  since  one  reads  in  opposite  directions,  the  errors 
made  in  going  up  will  neutralize,  to  a  great  extent,  those  made 
in  coming  down  the  stem. 

The  spaces  successively  filled  by  the  thread  of  mercury  are 
spaces  of  equal  capacity  or  volume,  and  how  these  are  to  be 
employed  in  the  calibration  of  the  stem  will  be  explained  by 
means  of  the  following  example  of  a  first  calibration  by  a 
student. 

A  thermometer  graduated  in  whole  degrees  and  ranging  from 
-  10°  to  360°  was  placed  in  steam  from  boiling  water  for  half  an 


48  QUANTITATIVE  EXERCISES 

hour.  Immediately  afterwards,  its  freezing  and  boiling  points 
were  determined  and  found  to  be  located  at  2°  and  102° 
respectively. 

The  height  of  the  barometer  corrected  was  764.28  mm., 
which  gave  for  the  boiling  point  of  water  100.16°. 

A  thread  spanning  35  divisions  in  the  lower  end  of  the  scale 
was  broken  off  and  manipulated  up  and  down  the  tube  in  the 
prescribed  manner.  The  following  readings  were  obtained. 

i  ii  in  iv  v 

TT-,  DIVISIONS  T.--,  DIVISIONS  MEAN  OF 

FILLED  FILLED  II  AND  IV 

0.0  0.0 

35.0  ....  35.0  35.0  ....  35.0       .         35.0 

70.2  ....  35.2  70.4  ....  35.4  35.3 

106.0  ....  35.8  106.0  ....  35.6  35.7 

142.0  ....  36.0  142.0  ....  36.0  36.0 
178.2  ....  36.2  178.4  ....  36.4  36.8 

215.1  ....  36.9  215.2  ....  36.8  36.9 
251.6  ....  36.5  251.8  ....  36.6  36.6 
288.0  ....  36.4  288.0  ....  36.2  36.3 
324.8  ....  36.8  324.8  ....  36.8  36.8 

The  whole  number  of  scale  divisions  filled  by  the  thread  when 
placed  nine  times  end  to  end  was  324.8,  while  the  average 


number  filled  was  -      -  =  36.09.     The  thread  was  therefore 

«7 

regarded  as  containing  36.09  volume  or  calibration  units. 

The  differences  between  the  averages  of  the  corresponding 
upward  and  downward  readings,  and  the  number  of  calibration 
units  in  the  thread,  multiplied  by  1,  2,  3  ...  9,  gave  the  cor- 
rections to  be  added  at  nine  points  on  the  scale  in  order  to 
convert  actual  into  calibrated  readings. 


THE  BAROMETER  AND  THE  THERMOMETER     49 

VI  VII  VIII 

CALIB.  READINGS  CORRECTIONS 

35.0 36.09  x  1  =    36.09  .......+  1.09 

70.3 36.09  x  2  =    72.18 1.88 

106.0 36.09x3  =  108.27 2.27 

142.0 36.09  x  4  =  144.36 2.36 

178.3 .  36.09  x  5  =  180.45 2.15 

215.2 36.09  x  6  =  216.54 1.34 

251.7 36.09  x  7  =  252.63 0.93 

288.0 36.09x8  =  288.72  ......  0.72 

324.8 36.09  x  9  =  324.80 0.00 

The  values  in  column  VIII  were  then  used  as  ordinates  to 
plot  on  cross-ruled  paper  a  curve  of  corrections,  the  thermometer 
scale  serving  as  the  axis  of  abscissas. 

It  will  be  seen  that  the  calibrated  readings  in  such  a  curve 
are  correct  at  certain  fixed  points,  in  this  case  at  nine  points  on 
the  scale  ;  further,  that  it  is  assumed  that  any  change  in  caliber 
between  two  adjacent  fixed  points  is  uniform  throughout  the 
whole  of  the  included  space.  This  assumption  is  usually  incor- 
rect, but  the  errors  resulting  from  it  are  not  large  because  they 
are  confined  to  the  particular  space  in  which  a  reading  falls. 
In  other  words,  errors  of  this  kind  do  not  accumulate  from 
space  to  space,  but  cease  altogether  at  the  fixed  points. 

In  the  case  of  a  thermometer  of  very  irregular  caliber  it  may, 
however,  be  desirable  to  increase  the  number  of  correct  ordinates 
for  the  curve,  and  thus  to  diminish  the  error  referred  to.  To 
do  this,  the  lower  end  of  the  thread  is  placed  about  halfway 
between  the  first  two  readings  and  the  calibration  is  repeated .  The 
number  of  correctly  established  points  will  thus  be  doubled,  and 
by  repeating  the  process,  starting  each  time  from  some  new  point 
on  the  scale,  the  number  may  be  increased  to  any  desired  extent. 

In  order  that  a  thermometer  which  has  been  calibrated  in  the 
manner  described  may  be  used  for  the  correct  determination  of 
temperature,  it  is  necessary  to  ascertain  the  temperature  equiva- 
lent of  the  calibration  unit.  In  the  case  of  the  instrument  used 


50  QUANTITATIVE  EXERCISES 

as  an  illustration,  the  correct  readings  found  on  the  curve  for 
the  freezing  and  boiling  points  were  2.20  and  104.2  respectively  ; 
i.e.  there  were  102.0  calibration  units  included  between  these 
two  points.  But  at  the  time  of  determining  the  freezing  and 
boiling  points  the  boiling  temperature  of  water  was  100.16°; 
therefore  the  temperature  equivalent  of  the  calibration  unit  was 

10016,  Or0.98196o. 

To  ascertain  the  temperature  corresponding  to  any  reading 
of  the  thermometer,  the  difference  between  it  and  the  zero 
point,  in  calibration  units,  is  to  be  multiplied  by  the  tempera- 
ture equivalent  of  the  unit.  When  a  thermometer  is  employed 
to  register  a  high  temperature  its  zero  point  should  be  deter- 
mined immediately  after,  rather  than  before,  the  experiment. 

Correction  for  the  Exposed  Part  of  a  Mercury  Column 

If  a  portion  of  the  mercury  in  the  stem  of  a  thermometer  is 
outside  of  the  area  whose  temperature  is  sought,  the  temperature 
of  the  exposed  part  is  different  from  that  of  the  mercury  in  the 
bulb.  The  column  is  accordingly  too  short  or  too  long,  and 
requires  a  correction ;  i.e.  there  must  be  added  to  or  subtracted 
from  its  length  the  amount  which  it  would  be  lengthened  or 
shortened  if  the  temperature  of  the  exposed  part  were  raised 
or  lowered  to  the  temperature  of  the  mercury  in  the  bulb.  The 
correction  to  be  applied  is 

(t  —  t')  n%  in  which 

t  is  the  temperature  observed  on  the  thermometer ; 
t',  the  mean  temperature  of  the  exposed  part  of  the  column, 
determined    by  placing  another  thermometer   near  its    middle 
point ; 

7i,  the  length  in  calibration  units  of  the  exposed  part ;  and 
7,  the  apparent  expansion  of  mercury  in  glass,  i.e.  the  differ- 
ence between  the  cubical-expansion  coefficients  of  mercury  and 
glass,  which  is  0.0001818  -  0.000025  or  0.0001568. 


THE  BAROMETER  AND  THE  THERMOMETER     51 

Obviously  t  is  not  exactly  the  temperature  of  the  mercury  in 
the  bulb.  The  correction  is  therefore  slightly  faulty ;  but  any 
desired  approximation  to  the  truth  may  be  secured  by  substitut- 
ing for  t  its  corrected  value  and  repeating  the  calculation. 

The  temperature  of  the  exposed  part  of  the  mercury  column 
cannot  be  determined  with  certainty.  It  is  assumed  to  have  the 
temperature  of  the  medium  immediately  surrounding  the  portion 
of  the  stem  which  it  occupies,  but  a  little  reflection  upon  the 
conditions  which  determine  its  temperature  will  convince  one 
that  the  assumption  is  not  altogether  warranted.  It  is  better, 
therefore,  if  practicable,  to  subject  the  whole  column  to  the 
same  temperature  conditions  as  the  bulb  rather  than  to  rely  on 
a  correction  for  an  exposed  fraction  of  the  mercury. 

The  Comparison  of  Thermometers 

When  two  thermometers  "are  to  be  compared,  great  care  must 
be  taken  to  insure  perfectly  uniform  conditions.  They  are  best 
compared  by  placing  them  in  the  vapors  of  boiling  liquids.  If 
the  comparison  is  made  by  immersing  them  in  a  liquid,  the  liquid 
should  be  stirred.  An  air  bath  is  not  to  be  trusted. 


Thermometers  filled  with  G-as 

When  the  ordinary  thermometer  in  which  there  is  a  vacuum 
above  the  mercury  is  heated  nearly  to  the  boiling  point  of  the 
metal  (357°),  the  column  in  the  stem  separates  in  conse- 
quence of  the  formation  of  vapor.  If,  on  the  other  hand,  the 
space  is  filled  with  a  gas  under  pressure,  the  boiling  point  of  the 
mercury  is  raised  and  the  thermometer  will  register  much  higher 
temperatures.  But  progress  in  this  direction  is  limited  by  the 
comparatively  low  temperatures  at  which  most  varieties  of  glass 
begin  to  soften.  Within  recent  years,  however,  a  borosilicate 
glass  with  high  fusing  point  has  been  produced  in  Jena  from 
which  thermometers  are  made  and  filled  with  carbon  dioxide 


52  QUANTITATIVE  EXERCISES 

under  a  pressure  of  17  or  18  atmospheres.    These  thermometers 
register  temperatures  up  to  550°. 

The  Beckmann  Thermometer 

A  thermometer  much  used  in  the  determination  of  molecu- 
lar weights  by  the  freezing-  and  boiling-point  methods  is  that 
devised  by  Beckmann,  Fig.  35.  In  this  instrument  the  bulb  is 
so  capacious  that  the  entire  scale  covers  not  more  than  five  or 
six  degrees,  permitting  a  graduation  of  the  stem  to  hundredths, 
and,  in  some  cases,  even  to  thousandths  of  a  degree.  In  order 
now  that  the  thermometer  may  be  used  for  a  considerable  variety 
of  temperatures,  notwithstanding  the  narrow  range  of  its  gradu- 
ation, a  reservoir  is  placed  in  its  upper  end  to  which  any  desired 
portion  of  the  mercury  in  the  bulb  may  be  transferred.  It  will 
be  seen  that  the  value  of  a  scale  division  in  such  an  instrument 
depends  on  the  amount  of  the  mercury  left  in  the  bulb.  The 
manner  of  using  the  Beckmann  thermometer  and  of  correcting 
its  readings,  when  a  portion  of  the  mercury  has  been  removed 
from  the  bulb  to  the  reservoir,  will  be  explained  in  the  chapter 
on  the  determination  of  molecular  weights. 

The  Air  Thermometer 

The  simplest  form  of  the  gas  thermometer,  Fig.  6,  consists 
of  a  glass,  porcelain,  or  platinum  bulb  connected  by  means  of  a 
capillary  tube  with  a  U-shaped  adjustable  column  of  mercury. 
Its  use  for  the  determination  of  temperature  is 
based  on  the  experience  that,  with  constant  pres- 
sure, a  gas  expands  very  nearly  uniformly  for  equal 
increments  of  temperature  (^3-  part  of  its  volume 
at  0°  for  each  degree);  or,  if  the  volume  is  con- 
stant, the  pressure  increases  in  the  same  ratio. 
In  practice  the  volume  is  kept  constant  and  the 
temperature  is  deduced  from  the  pressure.  For  the  measure- 
ment of  high  temperatures  the  bulb  is  filled  with  hydrogen  rather 


THE  BAROMETER  AND  THE  THERMOMETER     53 

than  air  because  under  heavy  pressures  the  former  gas  obeys  the 
law  with  greater  precision  than  the  latter. 

Measurements  of  temperature  by  the  mercury  and  by  the  gas 
thermometer  do  not  perfectly  agree  except  at  the  experimentally 
fixed  points,  —  the  freezing  and  boiling  points  of  water.  The 
reason  for  the  divergence  is  to  be  found  in  the  irregularity  of 
the  expansion  of  both  mercury  and  glass.  In  the  mercury  ther- 
mometer temperatures  are  measured  by  the  apparent  expansion 
of  mercury,  and  this  depends,  in  turn,  on  the  absolute  expansion 
of  both  the  metal  and  the  glass,  which  is  not  perfectly  uniform 
in  the  case  of  either  material.  Moreover,  different  varieties  of 
glass  exhibit  different  degrees  of  irregularity  in  respect  to  their 
expansion  coefficients ;  hence  thermometers  made  from  different 
kinds  of  glass,  though  carefully  calibrated,  will  not  perfectly 
agree  with  one  another  except  at  experimentally  fixed  points. 

A  difficulty  of  the  same  character  is  experienced  in  connec- 
tion with  the  gas  thermometer,  inasmuch  as  the  expansion  of  the 
material  of  the  bulb  as  well  as  that  of  the  gas  must  be  taken 
into  account.  The  former,  however,  is  so  minute  as  compared 
with  the  latter  that  the  mere  irregularities  in  the  expansion  of 
the  bulb  have  no  appreciable  effect  on  the  results. 

Since  the  expansion  of  a  gas,  especially  hydrogen,  is  much 
more  regular  than  that  of  either  mercury  or  glass,  and  since  the 
gas  thermometer  is  but  little  affected  by  the  irregularities  of  the 
expansion  of  its  bulb,  the  gas  rather  than  the  mercury  thermome- 
ter is  to  be  regarded  as  the  standard  instrument,  and  determi- 
nations of  temperature  by  the  latter  are  to  be  corrected  by  adding 
the  ascertained  divergences  between  the  two  classes  of  instru- 
ments. These  divergences  are  sometimes  positive  and  some- 
times negative  in  character.  They  are  to  be  found  in  tabular 
form  in  the  Physikalisch-Chemische  Tabellen  of  Landolt  and 
Boernstein. 


54  QUANTITATIVE  EXERCISES 

Alcohol  and  Toluene  Thermometers 

Owing  to  its  low  freezing  point  alcohol  has  been  much  used 
in  the  preparation  of  thermometers  for  temperatures  under 
the  solidifying  point  of  mercury.  It  is,  however,  an  exceed- 
ingly defective  material  for  the  purpose.  Its  rate  of  expansion 
increases  very  rapidly  with  rising  temperature  and  is  greatly 
affected  by  the  presence  of  small  quantities  of  water.  The 
amount  of  the  alcohol  which  adheres  to  the  wall  of  the  ther- 
mometer tube  increases  with  falling  temperature  to  such  an 
extent  that,  at  different  temperatures,  very  different  quantities 
of  the  liquid  are  employed  in  the  mere  wetting  of  the  glass.  It 
is  customary,  in  preparing  these  instruments,  to  fix  the  zero 
point  in  the  usual  way  and  to  locate  some  second  point  by  com- 
parison with  a  standard  thermometer.  By  means  of  these  two 
fixed  points  the  whole  scale  is  then  uniformly  graduated. 

When  the  alcohol  and  air  thermometers  are  compared,  large 
discrepancies  are  found.  The  divergence  increases  rapidly  with 
falling  temperature,  and  amounts  to  about  10°  at  — 100°. 

In  some  respects  toluene  is  superior  to  alcohol  as  a  material 
for  low-temperature  thermometers.  It  can  be  prepared  in  uni- 
formly pure  condition  so  that  the  instruments  agree  with  one 
another,  and  its  high  boiling  point  (110°)  makes  it  practicable 
to  locate  experimentally  both  of  the  usual  fixed  points  of  a 
thermometer,  i.e.  the  freezing  and  boiling  points  of  water. 
Nevertheless,  the  disagreement  with  the  air  thermometer  at  low 
temperatures  is  even  greater  in  the  case  of  the  toluene  than  in 
that  of  the  alcohol  thermometer. 


Determination  of  Temperatures  by  Means  of  Substances  of 
Known  Melting  Points 

It  is  often  necessary  in  the  laboratory,  when  the  circum- 
stances do  not  permit  the  use  of  a  thermometer  or  a  pyrometer, 
to  ascertain  within  narrow  limits  the  temperature  which  is 


THE  BAROMETER  AND  THE  THERMOMETER     55 

produced  by  a  given  heating  arrangement,  or  to  know  that 
the  highest  temperature  reached  during  an  experiment  lies 
between  two  limits.  In  such  cases  substances  of  known  fusing 
points  can  generally  be  used  with  advantage.  If  it  is  found, 
for  instance,  that  the  temperature  attained  under  the  given  con- 
ditions suffices  for  the  fusion  of  sodium  chloride  but  not  for  that 
of  potassium  carbonate,  then  it  is  known  that  the  highest  tem- 
perature reached  is  above  776°  and  below  835°.  If  the  former 
fuses  very  readily  while  the  latter  shows  no  signs  of  having 
been  in  the  least  degree  softened,  it  is  reasonable  to  assume  that 
the  temperature  was  about  800°. 

In  this  connection  should  be  mentioned  the  so-called  "  Seger 
cones."  These  are  triangular  truncated  pyramids  six  centimeters 
in  height  which  are  made  up  of  mixtures  of  silicates  of  gradu- 
ally diminishing  fusibility.  They  are  much  used  in  the  baking 
of  pottery  and  are  often  serviceable  in  the  laboratory. 

The  following  table  gives  the  number  and  the  melting  point 
of  each  one  of  the  series  of  Seger  cones. 

022  =  590°  09  =    970°  4  =  1210°         16  =  1450°         28  =  1690° 


021  =  620 

08=  990 

5  =  1230 

17  =  1470 

29  =  1710 

020  =  650 

07  =  1010 

6  =  1250 

18=1490 

30  =  1730 

019  =  680 

06  =  1030 

7  =  1270 

19  =  1510 

31  =  1750 

018  =  710 

05  =  1050 

8  =  1290 

20  =  1530 

32  =  1770 

017  =  740 

04  =  1070 

9  =  1310 

21  =1550 

33  =  1790 

016  =  770 

03  =  1090 

10  =  1330 

22  =  1570 

34  =  1810 

015  =  800 

02  =  1110 

11  =  1350 

23=1590 

35  =  1830 

014  =  830 

01  =1130 

12  =  1370 

24  =  1610 

36  =  1850 

013  =860 

1  =  1150 

13  =  1390 

25  =  1630 

37  =1870 

012  =  890 

2  =  1170 

14  =1410 

26  =  1650 

38  =  1890 

Oil  =  920 

3  =  1190 

15=1430 

27  =  1670 

39  =  1910 

010  =  950 

Pyrometers 

Of  the  many  kinds  of  pyrometers  which  have  been  proposed 
or  employed  for  the  measurement  of  high  temperatures,  only  two 
have  any  special  interest  for  the  chemist,  —  the  electric-resistance 


56  QUANTITATIVE  EXERCISES 

pyrometer  and  the  thermoelectric  pyrometer.  The  former  is 
based  on  the  fact  that  the  electric  resistance  of  metals  varies 
with  the  temperature.  Platinum  is  the  metal  employed  in  its 
construction.  The  best  known  instrument  of  this  type  is  that 
of  Callender.  The  thermoelectric  pyrometer,  on  the  other 
hand,  utilizes  the  electro-motive  force  which  is  developed  when 
a  junction  of  two  different  metals  or  alloys  is  heated.  The  best 
known  instrument  of  the  second  type  is  that  of  Le  Chatelier. 
The  metals  employed  in  its  construction  are  platinum  and  an 
alloy  of  platinum  with  ten  per  cent  of  rhodium.  The  electro- 
motive force  which  is  developed  when  the  junction  of  the  two 
wires  is  heated  is  measured  by  means  of  a  sensitive  galvanometer. 


CHAPTER  III 

THE  CALIBRATION  OF  EUDIOMETERS   AND  THE 
MEASUREMENT  OF  GASES 


EXERCISE  VII 
CALIBRATION  OF  A  EUDIOMETER 

I.   PURIFICATION  OF  THE  MERCURY 

The  apparatus  represented  in  Fig.  7  is  required.  The  large 
straight  tube  should  have  a  length  of  not  less  than  1^  meters 
and  an  internal  diameter  of  15-20  mm.  The  vertical  distance 
between  the  two  ends  of  the  doubly  bent  small  tube 
should  be  about  one-tenth  the  length  of  the  larger 
tube.  The  straight  tube  which  is  attached  to  the  stem 
of  the  funnel  is  to  be  drawn  down  at  the  lower  end  to 
a  minute  orifice  which  will  just  permit  the  mercury, 
when  under  some  pressure,  to  pass  through  it  in  a  fine 
stream. 

With  the  receiving  vessel  on  the  floor  and  the  clamp- 
ing stand  on  the  table,  fix  the  apparatus  at  the  side  of 
the  table.  Pour  in  more  than  enough  mercury  to  fill  the 
bent  tube  at  the  bottom,  and  then  a  dilute  solution  of 
ferric  chloride  (1 : 15  or  20)  until  the  larger  tube  is  nearly 
full.  Remove  from  the  receiver  the  mercury  crowded  out  of 
the  apparatus  by  the  pressure  of  the  liquid  above,  and  pass  the 
mercury  to  be  cleansed  through  the  funnel  into  the  solution  of 
ferric  chloride.  The  operation  is  to  be  repeated  until  the  metal 
is  sufficiently  pure  for  eudiometric  purposes,  i.e.  until,  after 
washing  and  drying,  it  shows  no  disposition  to  "  tail  out "  when 

67 


58  QUANTITATIVE   EXERCISES 

running  along  a  clean  glass  surface.  If  the  solution  of  ferric 
chloride  becomes  green  in  color,  it  must  be  replaced  by  a  fresh 
one.  If  it  is  too  concentrated,  the  mercury  shows  a  disincli- 
nation to  collect  at  the  bottom  in  a  sufficiently  liquid  form,  and 
the  apparatus,  especially  the  small  orifice  at  the  lower  end  of  the 
funnel,  becomes  clogged. 

The  frequently  persistent  refusal  of  finely  divided  mercury  to 
coalesce  is  due  to  a  coating  of  foreign  matter  which  may  be 
either  solid  or  in  solution,  and  the  difficulty  is  to  be  overcome 
only  by  removing  the  covering  which  prevents  contact  between 
the  minute  globules.  This  may  sometimes  be  accomplished  by 
rubbing  the  material  with  a  pestle  or  by  warming  and  agitating 
it  with  water  or  hydrochloric  acid.  But  the  surest  way  of 
accomplishing  the  object  is  to  force  the  mercury  through  finely 
woven  cloth  or  a  piece  of  chamois  skin. 

Replace  the  ferric  chloride  by  water  and  wash  the  mercury 
thoroughly,  renewing  the  water  as  often  as  may  be  thought 
necessary.  Dry  the  mercury  by  wiping  its  surface  with  filter 
paper  and  by  passing  it  through  filters  in  which  a  large  number 
of  pin  holes  have  been  made. 

Line  a  porcelain  dish  with  several  sheets  of  strong  filter 
paper,  place  the  mercury  on  the  upper  one,  and  dry  still  further 
at  105°  in  an  air  bath  under  a  hood.  The  drying  may  be  some- 
what facilitated  by  placing  glass  rods  or  tubes  between  the 
paper  and  the  porcelain  dish  in  such  a  manner  as  to  keep  the 
two  quite  apart. 

The  advantage  of  this  method  of  purification  over  those  in 
which  the  mercury  is  simply  agitated  with  the  cleansing  liquid 
lies  in  the  very  fine  subdivision  of  the  metal  which  is  effected 
at  the  orifice  of  the  funnel.  The  purification  can  take  place 
only  at  the  surface  of  contact  between  the  two  liquids ;  hence 
the  desirability  of  increasing  this  surface  to  the  greatest  possi- 
ble extent. 


THE   CALIBRATION   OF   EUDIOMETERS  59 

Solutions  of  other  substances  which  readily  attack  the  impuri- 
ties, but  act  only  slowly  or  not  at  all  on  the  mercury,  may  be 
used  in  the  place  of  the  ferric  chloride. 

A  supply  of  dry  and  sufficiently  pure  mercury  may  be  main- 
tained by  keeping  the  metal,  when  not  in  use,  under  strong 
sulphuric  acid  to  which  mercurous  sulphate  has  been  added. 
The  foreign  metals,  excepting  silver,  gold,  and  platinum,  are 
converted  into  sulphates.  The  apparatus  which  serves  as  a 
reservoir  must  be  of  such  construction  that  the  mercury  may 
be  drawn  off  at  the  bottom  and  returned  at  the  top.  Ordina- 
rily this  apparatus  consists  of  a  glass  globe  with  a  tube  and 
stopcock  underneath  and  a  neck  for  a  stopper  above.  A  large 
separating  funnel  may  be  used  for  the  purpose.  The  neck 
is  closed  with  a  doubly  perforated  stopper  through  which  are 
passed  a  funnel  with  a  stopcock  for  the  introduction  of  the 
mercury,  and  a  calcium  chloride  tube  to  dry  the  air  entering 
the  apparatus. 

Distillation  in  a  vacuum  —  for  which  a  variety  of  automatic 
stills  have  been  devised  —  is  also  an  excellent  method  for  the 
purification  of  mercury. 

Many  of  the  metals  when  amalgamated  oxidize  very  easily  in 
contact  with  the  air,  giving  rise  to  gray-colored  films  upon  the 
surface  of  the  mercury.  The  appearance  of  such  films  on  mer- 
cury exposed  to  the  air  is,  therefore,  to  be  regarded  as  an  evi- 
dence of  impurity,  while  their  absence  is  a  sign  of  some  value 
of  the  purity  of  the  metal.  The  color  of  the  films  is  due  to 
the  admixture  of  mercury  in  the  metallic  condition,  and  is  not, 
as  is  often  supposed,  the  result  of  the  formation  of  gray  sub- 
oxides  of  the  metals  attacked. 

Advantage  may  be  taken  of  the  fact  that  some  metals  when 
dissolved  in  mercury  are  easily  attacked  by  free  oxygen,  to 
purify  mercury  by  means  of  the  air.  For  this  purpose  currents 
of  air  are  forced  through  the  metal ;  or,  better,  the  metal  is 
allowed  to  fall  through  the  air  in  fine  streams, 


60  QUANTITATIVE  EXERCISES 

II.    CALIBRATION  OF  THE  EUDIOMETER 

Select  a  eudiometer  with  a  millimeter  graduation  extending 
over  not  less  than  70  centimeters.  Compare  the  graduation, 
which  is  sometimes  grossly  incorrect,  with  that  of  some  stand- 
ard supposed  to  be  reliable,  e.g.  with  that  of  the  barometer. 
Examine  it  with  a  magnifying  glass  for  the  minute  cracks 
which  are  apt  to  appear  where  the  platinum  wires  pass  through 
the  glass.  Fill  it  with  mercury,  taking  care  to  dislodge  all  air 
bubbles  which  become  entangled  between  the  mercury  and  the 
wall  of  the  tube.  This  may  be  accomplished  with  the  aid  of  a 
long  whalebone,  or  by  inclining  the  tube  from  time  to  time 
while  filling  and  slowly  returning  it  again  to  the  vertical  posi- 
tion. The  lodging  of  air  bubbles  against  the  glass  may  be 
avoided  altogether  by  filling  very  slowly  and  without  interrup- 
tion through  a  small  glass  tube  which  reaches  to  the  bottom  of 
the  eudiometer.  For  this  purpose  it  is  necessary  to  have,  placed 
above  the  eudiometer,  a  supply  of  mercury  whose  flow  through 
the  tube  can  be  properly  regulated. 

Invert  the  filled  eudiometer  in  a  mercury  trough  having 
transparent  sides.  Fix  it  in  a  cork-lined  clamp,  using  plumb 
lines  suspended  from  the  ceiling  to  determine  when  its  position 
is  vertical.  If  the  length  of  the  tube  is  greater  than  a  barometric 
height,  determine  the  length  of  the  column  in  the  eudiometer 
from  the  level  of  the  mercury  in  the  trough  to  the  top  of  the 
meniscus ;  also  the  height  of  the  barometer,  using  the  telescope 
for  both  readings.  After  several  hours  repeat  the  readings. 
If  the  height  of  the  barometer  and  that  of  the  columns  are 
unchanged,  or  if  both  have  risen  or  fallen  alike,  the  eudiometer 
does  not  leak.  If,  on  the  other  hand,  the  tube  is  shorter  than 
a  barometric  height,  the  existence  of  a  leak  will  be  detected  by 
the  accumulation  of  air  in  the  top  of  the  tube. 

Eudiometers  into  which  platinum  wires  have  been  fused  for 
the  explosion  of  gas  mixtures  are  often  ruined  in  consequence 
of  the  formation  —  especially  at  the  time  of  an  explosion  —  of 


THE  CALIBRATION  OF  EUDIOMETERS  61 

cracks  in  the  glass  around  the  wires.  The  fact  that  these  can- 
not be  detected  by  the  eye,  even  when  aided  by  a  lens,  is  never 
a  certain  proof  of  the  soundness  of  the  tube.  The  cause  of  the 
difficulty  is  a  slight  inequality  in  the  expansion  coefficients  of 
glass  and  platinum. 

The  calibrating  cup,  Fig.  8,  is  used  for  the  introduction  into 
the  eudiometer  of  equal  volumes  of  mercury.  The  capacity  of 
the  cup  should  not,  in  general,  exceed  5  cc.  Its  edge  is  ground 
to  a  plane  making  a  right  angle  with  its  axis.  The  covering  glass 
is  also  ground  on  one  side,  while  the  other  side  is  provided  with 
a  rubber  ring  —  cemented  to  the  glass  —  through  which  the 
thumb  may  be  slipped.  This  arrangement  enables  the  operator 
to  manipulate  the  cup  and  its  cover  with  one  hand. 

As  a  reservoir  for  the  supply  of  mercury,  a  separating  funnel, 
or  even  a  burette,  may  be  used.  It  is  necessary  only  that  the 
delivery  tube  of  the  vessel  should  be 
of  small  bore  and  of  sufficient  length 
below  the  stopcock  to  reach  the  bot- 
tom of  the  calibrating  cup. 

Bring  the  cup  under  the  reservoir 

so  that  the  outlet  of  the  latter  rests  upon  the  bottom  of  the  former 
and  slowly  fill  with  mercury,  lowering  the  cup  as  the  filling 
progresses,  but  not  enough  to  expose  the  end  of  the  delivery 
tube  until  the  cup  is  full.  In  this  way  the  lodging  of  air  bubbles 
between  the  mercury  and  the  glass  will  be  avoided.  Bring  the 
ground  glass  plate,  which  is  carried  on  the  thumb  of  the  hand 
holding  the  cup,  vertically  down  upon  the  surface  of  the  mercury 
and  gently  rub  it  from  side  to  side  to  remove  superfluous  metal. 
Pour  the  cupful  through  a  funnel  with  a  long  stem  of  small  bore 
into  the  eudiometer.  It  will  probably  be  found  that  air  bubbles 
have  lodged  between  the  glass  and  mercury,  also  that  minute 
globules  of  the  metal  have  attached  themselves  to  the  glass  for 
a  considerable  distance  up  the  tube.  The  former  must  be 
removed  and  the  latter  added  to  the  main  body  of  the  mercury. 
Cover  the  hands  with  towels  or  mittens.  Incline  the  eudiometer 


62  QUANTITATIVE   EXERCISES 

with  the  closed  end  resting  on  the  table  and  then  revolve  it 
to  collect  the  scattered  globules.  Finally,  bring  the  tube  very 
slowly  to  the  vertical  position. 

The  detached  globules  may  also  be  gathered  up  and  the  air 
bubbles  released  with  the  aid  of  a  long  piece  of  whalebone. 
In  this  way  the  danger  of  warming  the  tube  by  handling  is 
avoided. 

Fix  the  eudiometer  in  a  vertical  position  with  the  aid  of  the 
plumb  lines  and  read  with  the  telescope  the  height  of  the  mer- 
cury to  the  top  of  the  meniscus.  Continue  the  filling  in  of  the 
mercury  and  the  reading  of  the  height  of  the  column  after  each 
addition  until  the  tube  has  been  filled  to  a  depth  of  not  less 
than  400  mm. 

It  is  obvious  that  the  temperature  of  the  mercury  must 
remain  very  nearly  constant  during  the  whole  of  the  time 
occupied  in  filling  the  eudiometer. 

At  some  time  during  the  calibration  two  cupfuls  of  mercury 
should  be  poured  into  weighing  glasses  and  set  aside  for  the 
purpose  of  determining  the  capacity  of  the  calibrating  cup. 
A  record  of  the  temperature  of  the  mercury  will  also  be 
required. 

The  use  which  is  to  be  made  of  the  readings  obtained  during 
the  filling  of  the  eudiometer  will  be  illustrated  by  means  of  an 
example. 


THE   CALIBRATION   OF   EUDIOMETERS  63 

1  II                 III  IV  V 

21  596.4  28.4  28.54  x  21  =  599.34         difference  =  +  2.94 

20  568.0  28.9  "  x  20  =  570.80  "  =+2.80 

19  539.1  28.9  «  x  19  =  542.26  «  =+3.25 

18  510.2  29.0  «  x  18  =  513.72  «  =+3.52 

17  481.2  29.1  "  x  17  =  485.18  "  =+3.98 

16  452.1  29.1  "  x  16  =  456.64  «  =+4.54 

15  423.0  29.1  «  x  15  =  428.10  "  =+5.10 

14  393.9  29.1  «  x  14  =  399.56  «  =+5.66 

13  364.8  29.3  «  x  13  =  371.02  «  =+6.22 

12  335.5  29.2  «  x  12  =  342.48  «  =+6.98 

11  306.3  28.8  «  x  11  =  313.94  «  =+7.64 

10  277.5  28.4  «  x  10  =  285.40  '"  =+7.90 

9  249.1  28.3  «  x    9  =  256.86  «  =+7.76 

8  220.8  28.5  "  x    8  =  228.32  «  =  +  7.52 

7  192.3  28.0  «  x    7  =  199.78  «  =+7.48 

6  164.3  27.9  «  x    6  =  171.24  «  =+6.94 

5  136.4  27.9  "  x    5  =  142.70  «  =+6.30 

4  108.5  27.7  "  x    4  =  114.16  «  =+5.66 

3  80.8  27.8  «x    3=    85.62  «  =+4.82 

2  53.0  27.5  «  x    2=    57.08  «  =  +  4.08 
1  25.5  «  x    1=    28.54  «  =+3.04 

In  column  I  the  cupfuls  of  mercury  are  numbered  from  1  to 
21  in  the  order  in  which  they  were  poured  into  the  tube. 
Column  II  gives  the  successive  readings  on  the  graduation  of 
the  eudiometer,  and  column  III  the  difference  between  each 
reading  and  the  one  which  precedes  it,  i.e.  the  number  of 
millimeter  divisions  filled  by  the  various  cupfuls.  It  will  be 
observed  that  the  equal  volumes  of  mercury  fill  quite  different 
fractions  of  the  tube's  length,  showing  considerable  irregularity 
in  the  caliber  of  the  eudiometer.  The  first  cupful  fills  a  much 
smaller  number  of  divisions  than  any  one  of  the  others,  but  this 
is  owing  to  the  fact  that  the  graduation  is  not  extended  to  the 
end  of  the  tube. 

If  25.5  (the  first  reading)  is  subtracted  from  596.4  (the  last), 
it  is  found  that  twenty  cupfuls  of  mercury  fill  a  space  570.9  mm. 
in  length.  The  average  length  of  tube  filled  by  a  cupful  is 


64  QUANTITATIVE  EXERCISES 

therefore     ^  ,  or  28.54  mm.     For  this  reason  it  was  decided 

to  regard  the  cup  as  containing  28.54  volume  or  calibration 
units.  It  is  obvious  that  the  capacity  of  the  tube,  from  the 
closed  end  to  the  various  points  reached  by  the  different  cup- 
fuls  of  mercury,  can  be  found,  in  terms  of  the  calibration  unit, 
by  multiplying  28.54  by  the  number  of  cupfuls  which  fill  the 
tube  to  these  points.  This  has  been  done  for  each  cupful  under 
IV.  Column  V  contains  the  quantities  which  must  be  added 
to  the  actual  readings  in  order  to  obtain  their  equivalents  in 
calibration  units.  To  obtain  the  value  of  other  readings  than 
those  recorded  in  column  II,  a  curve  is  employed.  To  con- 
struct this,  the  millimeter  graduation  on  the  tube  is  made  the 
axis  of  abscissas,  and  the  quantities  to  be  added  to  the  actual 
readings,  i.e.  those  recorded  under  V,  the  ordinates.  The  curve 
is  completed  by  drawing  straight  lines  between  the  adjacent 
points  thus  established.  It  will  be  seen  that  in  this  case,  as  in 
the  calibration  of  the  thermometer  by  a  similar  method,  only  a 
limited  number  of  points  in  the  curve  are  established  with  cer- 
tainty. The  assumption  that  all  other  points  in  the  curve  lie 
in  the  straight  lines  joining  these  —  in  other  words,  that  the 
change  in  the  caliber  of  the  tube  is  always  uniform  between 
two  successive  established  points  —  is  not  any  more  correct  in 
the  case  of  a  eudiometer  than  in  that  of  a  thermometer.  In 
the  calibration  of  a  thermometer,  as  already  shown,  the  number 
of  correctly  established  points  in  the  curve  can  be  multiplied 
to  any  desired  extent  without  diminishing  the  length  of  the 
thread.  In  the  case  of  a  eudiometer,  on  the  contrary,  the  size 
of  the  cupful  —  the  equivalent  of  the  thread  —  must  be  dimin- 
ished if  a  closer  calibration  is  required. 

The  curve  should  be  plotted  upon  cross-ruled  paper  with 
lines  one  millimeter  or  one-tenth  of  an  inch  apart,  in  such  a 
manner  that  each  space  on  the  horizontal  lines  represents  a  mil- 
limeter division  of  the  graduation  on  the  tube,  and  each  space 
on  the  vertical  lines,  one-tenth  of  a  unit  of  the  correction  to 


THE  CALIBRATION  OF  EUDIOMETERS 


65 


be  made.  It  is  advisable,  in  order  to  keep  the  curve  of  the  cor- 
rections near  the  line  representing  the  eudiometer,  to  indicate 
the  addition  of  whole  numbers,  and  to  employ  the  curve  only 
for  the  fractional  parts. 

The  following  is  the  usual,  though  less  satisfactory,  method 
of  elaborating  the  calibration  data.  It  will  be  seen  that  the 
second  cupful  of  mercury  which  was  poured  into  the  tube  occu- 
pied a  space  27.5  mm.  in  length.  If  now  28.54  —  the  number 
of  calibration  units  in  the  cup  —  is  divided  by  this  number, 
there  will  be  obtained,  in  terms  of  the  calibration  unit,  the 
average  value  of  a  millimeter  division  between  the  first  and 
second  readings.  The  value  of  the  millimeter  spaces  in  all 
other  parts  of  the  tube  may  be  obtained  in  the  same  way.  The 
results  of  twenty  such  divisions  are  given  under  VI. 


VI 

V.R. 

COB.  V. 

V.R. 

COB.  V. 

21 

1.0049 

26 

29.06 

46 

49.81 

20 

0.9875 

27 

30.10 

47 

50.85 

19 

0.9875 

28 

31.13 

48 

51.89 

18 

0.9841 

29 

32.17 

49 

52.93 

17 

0.9808 

30 

33.21 

50 

53.97 

16 

0.9808 

31 

34.25 

51 

55.00 

15 

0.9808 

32 

35.29 

52 

56.04 

14 

0.9808 

33 

36.32 

53 

57.08 

13 

0.9741 

34 

37.36 

54 

58.11 

12 

0.9774 

35 

38.40 

55 

59.13 

11 

0.9910 

36 

39.44 

56 

60.16 

10 

1.0049 

37 

40.47 

57 

61.19 

9 

1.0085 

38 

41.51 

58 

62.21 

8 

1.0014 

39 

42.55 

59 

63.24 

7 

1.0193 

40 

43.59 

60 

64.27 

6 

1.0229 

41 

44.63 

etc. 

etc. 

5 

1.0229 

42 

45.66 

4 

1.0303 

43 

46.70 

3 

1.0266 

44 

47.74 

2 

1.0378 

45 

48.78 

60  QUANTITATIVE  EXERCISES 

The  volume  capacity  of  the  tube  to  25.5,  the  first  reading,  is 
28.54  calibration  units,  and  if  there  is  added  to  this  0.5189,  — 
one  half  the  value  of  the  millimeter  spaces  in  that  part  of  the 
tube,  —  the  corrected  volume  to  the  twenty-sixth  division  of  the 
scale  will  be  obtained,  i.e.  29.06.  The  volume  to  the  twenty- 
seventh,  and  the  volumes  to  the  succeeding  divisions  as  far  as 
the  fifty-third,  are  found  by  adding  the  number  1.0378.  From 
the  fifty-third  to  the  eighty-first  division  the  number  1.0266  is 
to  be  added,  etc.  The  usual  form  of  tabulation  is  shown  under 
V.R.  and  Cor.  V.,  which  signify  volume  read  and  correct  volume 
respectively. 

It  will  be  found,  on  contrasting  the  two  methods  of  dealing 
with  the  calibration  data,  that  the  first  is  less  laborious  and 
more  correct  than  the  second.  It  also  presents  the  results  in' a 
form  which  is  more  convenient  for  use.  The  first  assumes  that 
any  change  in  the  caliber  of  the  tube,  within  the  space  filled  by 
a  cupful  of  mercury,  is  gradual  from  reading  to  reading ;  while 
the  second  supposes  the  caliber  of  the  tube  to  be  uniform 
between  two  successive  readings,  i.e.  that  changes  of  caliber 
occur  only  where  one  cupful  ends  and  another  begins.  The 
latter  assumption  is  obviously  more  erroneous  than  the  former. 
The  corrections  for  the  fractional  parts  of  the  readings  must  be 
found  by  calculation  when  the  second  method  is  employed, 
while  the  curve  enables  one  to  estimate  them  to  the  second 
decimal  place  by  the  eye. 

It  will  be  seen  that  neither  method  provides  for  the  measure- 
ment of  a  gas  volume  smaller  than  the  cup  itself.  If  it  is 
desired  to  find  the  volumes  of  smaller  quantities  of  gas,  a 
smaller  calibrating  cup  must  be  used. 

III.  DETERMINATION  OF  THE  VALUE  OF  THE  MENISCUS 

A  correction  for  the  meniscus  is  to  be  applied  to  any  volume 
of  gas  measured  over  a  liquid.  If  the  liquid  is  mercury,  the 
meniscus  is  convex  and  the  correction  is  to  be  added;  if  it  is 


THE  CALIBRATION  OF  EUDIOMETERS 


67 


water  or  an  aqueous  solution,  the  meniscus  is  concave  and  the 
correction  is  to  be  subtracted. 

Suppose,  when  the  mercury  is  poured  into  the  eudiometer  for 
calibration  purposes,  that  the  first  cupful  gives  a  reading  of 
20  mm.,  as  shown  in  Fig.  9.  Then  imagine  the  tube  to  be 
inverted  and  to  contain  enough  gas  collected  over  mercury  to 
give  the  same  reading,  as  shown  in  Fig.  10.  It  is  evident,  not- 
withstanding the  fact  that  the  readings  are  identical,  that  the 
volume  of  the  gas  in  the  second  case  is  greater  than  that  of 
the  mercury  in  the  first  by  the  space  aa\  and  that  whenever 


FIG.  9 


FIG.  10 


FIG.  11 


any  quantity  of  gas  is  measured,  it  will  be  necessary  to  add  this 
space  to  its  apparent  volume  in  calibration  units.  This  is  known 
as  the  correction  for  the  double  meniscus,  and  the  magnitude  of 
the  correction  is  now  to  be  determined. 

Fix  the  eudiometer  in  a  vertical  position  and  partially  fill  it 
with  mercury.  Read  on  the  graduation  of  the  tube,  with  the 
telescope,  the  highest  point  in  the  meniscus.  Pour  over  the 
top  of  the  mercury  a  few  drops  of  a  dilute  solution  of  mercuric 
chloride.  In  a  few  moments  the  end  of  the  mercury  column 
will  lose  its  convex  form  and  become  horizontal.  Read  again. 
The  difference  between  the  two  readings  is  the  correction  for  the 


68  QUANTITATIVE  EXERCISES 

single  meniscus,  and  twice  this  is  the  quantity  always  to  be 
added  to  a  gas  volume  measured  in  the  tube. 

The  correction  value  of  the  meniscus  varies  with  the  diame- 
ter of  the  tube  and  the  nature  of  the  liquid.  The  following 
table  gives  the  amount  of  the  correction,  as  determined  by 
Bunsen,  for  tubes  of  various  diameters. 


DIAMETER  OF  TUBE 

WATER 

7  PER  CENT  Na  OH 

MERCURY 

14  mm. 

1.10  mm. 

0.70  mm. 

0.57  mm. 

15 

1.03 

0.63 

0.53 

16 

0.97 

0.57 

0.48 

17 

0.91 

0.51 

0.44 

18 

0.87 

0.47 

0.38 

19 

0.84 

0.44 

0.32 

20 

0.82 

0.42 

0.26 

21 

0.80 

0.40 

0.20 

IV.  DETERMINATION  OF  THE  VOLUME  OF  THE  CALIBRATION 

UNIT 

For  most  gas  analytical  purposes  an  arbitrary  unit  of  volume, 
like  the  calibration  unit,  is  sufficient ;  but  it  frequently  happens 
that  a  knowledge  of  the  volume  of  a  gas  in  cubic  centimeters 
is  required.  It  is  therefore  desirable  in  every  case  to  determine 
the  value  of  the  calibration  unit  in  terms  of  the  cubic  centi- 
meter. To  do  this,  weigh  the  two  cupfuls  of  mercury  which 
were  set  aside  while  calibrating  the  tube,  and  correct  half  the 
sum  of  the  two  weights  for  displacement  of  air.  The  volume 
of  the  calibration  unit  will  then  be  found  by  substituting  the 
value  of  #,  £,  and  V  in  the  following  equation : 

1  +  0.0001818  t 
C=ff~    13.5956  V      'mwhlch 

c  is  the  value  to  be  found ; 
g  is  the  weight  of  mercury ; 

0.0001818  is  the  mean  expansion  coefficient  of  mercury 
between  0°  and  30°; 


THE  CALIBRATION  OF  EUDIOMETERS  69 

t  is  the  temperature  of  the  mercury  when  measured  off; 
13.5956,  the  weight  of  one  cubic  centimeter  of  mercury  at  0° ; 
and  F,  the  number  of  calibration  units  in  the  cup. 

Measurement  of  Gases  over  Water 

We  have  now  to  consider  under  what  restrictions  a  tube 
which  has  been  calibrated  with  mercury  may  be  used  for  the 
measurement  of  gases  over  water  or  other  liquids  than  mercury. 
If  a  quantity  of  gas  is  introduced  into  a  eudiometer  filled  with 
water,  there  are  two  conditions  affecting  its  measurement  which 
require  attention :  first,  the  water,  in  descending  to  make  room 
for  the  gas,  leaves  a  film  upon  the  glass,  thereby  diminishing 
the  capacity  of  the  tube ;  second,  the  meniscus  differs  both  in 
character  and  value  from  that  of  mercury. 

There  are  various  ways  of  ascertaining  the  amount  of  the 
correction  to  be  made  for  the  film,  but  as  this  error  is  small  as 
compared  with  others  which  are  encountered  when  gases  are 
measured  over  any  liquid  except  mercury,  the  correction  is 
rarely  attempted. 

When  a  gas  is  collected  over  water  in  a  tube  which  has  been 
calibrated  with  mercury  the  correction  for  the  meniscus  is  nega- 
tive, and  amounts  to  the  difference  between  the  water  and  the 
mercury  meniscus.  Suppose,  Fig.  11,  in  calibrating  a  tube,  that 
the  first  cupful  had  filled  to  the  twentieth  division  on  the  grad- 
uation, and  that  afterwards  a  quantity  of  gas  giving  the  same 
reading  had  been  collected  over  water.  The  mercury  meniscus 
is  represented  by  the  line  a  a  a,  and  that  of  the  water  by  b  a  b. 
The  volume  of  the  mercury  is  known,  it  having  been  determined 
in  the  course  of  the  calibration ;  but  the  volume  of  the  gas  over 
water  is  less  than  that  of  the  mercury  by  the  space  cc,  which  is 
equal  to  the  difference  between  a  water  and  a  mercury  meniscus. 


70  QUANTITATIVE  EXERCISES 

The  Absorption  of  Gases  by  Liquids 

It  is  often  convenient  and  sometimes  necessary  to  collect  and 
measure  gases  over  other  liquids  than  mercury.  But  it  should 
be  borne  in  mind  that  the  absorption  and  diffusion  of  gases  are 
serious  obstacles  in  the  way  of  employing  such  liquids  when 
the  work  in  hand  demands  a  high  degree  of  accuracy;  and  one 
must  consider  in  situations  where  convenience  would  be  pro- 
moted by  the  use  of  another  liquid  than  mercury  whether  or  not 
the  resulting  unavoidable  errors  will  be  of  tolerable  magnitude. 
A  brief  recapitulation  of  some  of  the  more  important  facts  relat- 
ing to  the  absorption  of  gases  by  liquids  may  be  useful  in  this 
connection. 

1.  The  absorption  coefficient  of  a  gas  with  respect  to  any  par- 
ticular liquid  is  the  volume  of  the  gas,  measured  under  standard 
conditions  of  temperature  and  pressure,  which  a  unit  volume  of  the 
liquid  will  absorb  when  exposed  in  an  atmosphere  of  the  pure  gas. 

2.  The  Effect  of  Temperature.    The  power  of  liquids  to  retain 
gases  decreases  with  rising  temperature,  hence  changes  in  tem- 
perature are  followed  by  changes  in  absorption  coefficients  ;  but 
the  law  which  regulates  the  relation  of  absorption  coefficients  to 
temperature  is  not  known. 

3.  The  Effect  of  Pressure.    The  volume  of  a  gas  which  a  unit 
volume  of  a  liquid  will  absorb  is  independent  of  the  pressure. 
This  is  known  as  Henry's  Law.     But,  since  the  amount  of  mat- 
ter in  a  given  volume  of  gas  is  proportional  to  the  pressure,  the 
absorption  coefficients  are,  according  to  the  definition,  also  pro- 
portional to  the  pressure.     It  will  be  seen  that  the  term  absorp- 
tion coefficient  is  fully  defined  only  when  accompanied  by  a 
statement  of  both  temperature  and  pressure. 

4.  Absorption  from  a  Mixture  of  Gases.    From  a  mixture  of 
gases  each  constituent  will  be  absorbed  according  to  its  partial 
pressure,  i.e.  according  to  the  proportion  of  the  total  pressure 
which  is  due  to  its   presence.     Stated  in  another  way,  each 
constituent  will  be  absorbed  to  the  same  extent  that  it  would  be 


THE   CALIBRATION  OF  EUDIOMETERS  71 

if,  without  changing  the  volume,  all  the  other  components  of  the 
mixture  were  removed.  This  is  known  as  Dalton's  Law  of  Par- 
tial Pressures.  Its  application  to  specific  cases  will  be  made 
clearer  by  an  illustration.  At  0°  and  under  a  pressure  equal 
to  760  mm.  of  mercury  the  absorption  coefficients  of  nitrogen 
and  oxygen  are  0.02035  and  0.04114  respectively;  that  is, 
under  the  given  conditions,  a  liter  of  water  would  absorb 
20.35  cc.  of  gas  in  an  atmosphere  of  pure  nitrogen  and  41.14  cc. 
in  an  atmosphere  of  pure  oxygen.  But  suppose  the  liter  of 
water  is  exposed  under  the  same  conditions  of  temperature  and 
pressure  to  the  air,  which  is  a  mixture  of  four  volumes  of  nitro- 
gen with  one  of  oxygen.  According  to  the  law  of  partial  pres- 
sures the  water  will  absorb  |  of  0.02035  of  nitrogen  and  |  of 
0.04114  of  oxygen;  that  is,  after  saturation,  it  will  be  found  to 
contain  16.28  cc.  of  nitrogen  and  8.23  cc.  of  oxygen. 

The  embarrassing  effects  of  absorption  phenomena  upon  gas 
analytical  operations  when  other  liquids  than  mercury  are  used 
are  quite  obvious  ;  nevertheless  it  may  not  be  superfluous  to  call 
attention  to  a  few  of  them. 

Suppose  a  mixture  of  gases  is  required,  for  any  purpose,  to 
pass  through  a  liquid.  The  liquid  will  become  saturated  with 
the  various  constituents  of  the  mixture  in  accordance  with  the 
absorption  coefficient  of  each  and  the  law  of  partial  pressures. 
The  consequence  is  that  for  a  time  the  gas  which  passes  out  of 
the  liquid  will  differ  in  composition  from  that  which  entered. 
The  alteration  in  the  composition  of  the  gas  will  cease,  of 
course,  when  the  liquid  has  become  fully  saturated.  For  the 
same  reason,  a  mixture  of  gases  standing  over  a  liquid  will 
suffer  a  change  in  composition  unless  the  liquid  is  previously 
saturated  with  a  mixture  of  identical  composition.  The  prac- 
tice of  first  saturating  a  liquid  with  the  same  kind  of  gas  that  is 
afterwards  to  be  passed  through  it,  or  collected  over  it,  is  very 
common  in  gas  analysis  but  is  not  without  its  disadvantages. 
A  simple  example  will  illustrate  this.  Suppose  a  gas  containing 
#,  £>,  and  c  has  been  collected  over  water  previously  saturated 


72  QUANTITATIVE  EXERCISES 

with  some  of  the  same  kind  of  gas,  and  is  to  be  analyzed  by 
removing  the  various  constituents  in  turn  by  means  of  appro- 
priate absorbents  and  measuring  the  contraction  which  takes 
place  after  each  withdrawal.  If  other  gases,  e.g.  air,  are  kept 
out  of  the  water,  and  changes  of  temperature  do  not  bring  about 
a  change  in  the  relative  magnitudes  of  the  absorption  coeffi- 
cients of  the  different  components,  the  gas  would  maintain  its 
composition  indefinitely.  But  suppose  one  of  the  constituents, 
e.g.  a,  is  removed.  The  water  will  then  be  supersaturated  as 
far  as  a  is  concerned,  and  probably  undersaturated  as  regards  b 
and  c\  consequently  a  small  portion  of  b  and  c  will  enter  the 
water  while  the  residue  becomes  contaminated  with  a. 

Another  difficulty  which  is  encountered  when  other  liquids 
than  mercury  are  employed  for  the  isolation  of  gases  is  the  fact 
that  two  gases  cannot  be  permanently  separated  by  a  liquid 
which  is  capable  of  gas  absorption.  The  usual  source  of  trouble 
in  this  connection  is  the  air.  Suppose,  by  way  of  illustration, 
that  a  body  of  hydrogen  is  standing  in  a  eudiometer  over  water 
which,  as  usual,  is  also  in  contact  with  air.  From  the  one  side 
the  water  will  saturate  itself  with  hydrogen,  and  from  the  other 
with  the  oxygen  and  nitrogen  of  the  air ;  and  the  law  of  partial 
pressures  will  require  the  hydrogen  to  pass  out  of  the  water 
into  the  atmosphere,  while  the  constituents  of  the  air,  for  the 
same  reason,  enter  the  space  occupied  by  the  hydrogen.  Theo- 
retically this  exchange  would  continue  until  the  two  gases 
became  identical  in  composition;  but  since  the  volume  of  the 
air  is  infinitely  greater  than  that  of  the  hydrogen,  the  final  result 
would  be  a  complete  replacement  of  the  latter  by  the  former. 

The  facts  cited  above  will  suffice,  it  is  hoped,  to  impress  upon 
the  student  the  need  of  great  circumspection  in  dealing  with 
gases.  In  this  field,  as  in  all  other  kinds  of  quantitative  work, 
when  a  given  course  of  procedure  suggests  itself  or  is  recom- 
mended, one  should  first  of  all,  and  as  a  matter  of  habit,  scruti- 
nize the  sources  of  error  and  consider  the  means  by  which  they 
may  be  avoided  or  minimized. 


THE  CALIBRATION  OF  EUDIOMETERS  73 

The  Correction  of  G-as  Volumes  for  Pressure,  Temperature, 
and  Water  Vapor 

The  space  which  any  given  mass  of  gas  will  occupy  depends 
on  the  pressure  to  which  it  is  subjected,  its  temperature,  and 
the  amount  of  water  vapor  which  it  contains.  Hence,  for  the 
purpose  of  comparing  gases  with  respect  to  mass,  it  is  necessary 
to  reduce  the  observed  volumes  to  those  which  the  gases  would 
have  if  measured  under  the  same  conditions. 

I.  The  Correction  for  Pressure 

When  a  given  mass  of  gas  is  measured  under  different  pres- 
sures, without  change  of  temperature,  its  various  volumes  are 
found  to  be  inversely  proportional  to  the  pressures.  That  is, 
if  the  pressure  is  doubled,  the  volume  will  be  halved;  or  if 
the  pressure  is  halved,  the  volume  will  be  doubled,  etc.  Stated 
in  another  way,  the  product  of  the  pressure  and  the  volume  is  a 
constant.  This  relation  of  the  volume  of  a  gas  to  the  pressure 
upon  it  is  known  as  Boyle's  law  —  also  as  the  law  of  Mariotte. 
Gases  which  cannot  be  condensed  to  liquids  except  at  very  low 
temperatures  —  like  hydrogen,  oxygen,  nitrogen,  carbon  monox- 
ide, and  methane  —  obey  the  law  within  ordinary  ranges  of 
pressure  but  deviate  from  it  more  or  less  under  high  pressures. 
Gases  easily  condensed  to  liquids,  likewise  the  vapors  of  liquids, 
do  not  conform  to  the  law  until  heated  considerably  above  their 
temperatures  of  liquefaction. 

If  a  gas  is  confined  over  a  column  of  liquid  which  is,  in  turn, 
in  free  communication  with  the  air,  the  pressure  of  the  gas  and 
that  of  the  column  of  liquid  are  together  equal  to  the  pressure 
of  the  air.  Hence,  in  order  to  find  the  pressure  of  the  gas,  we 
must  subtract  the  pressure  of  the  column  of  liquid  from  the 
height  of  the  barometer.  If  the  liquid  employed  to  isolate 
the  gas  is  mercury,  the  difference  between  its  height,  corrected 
to  0°,  and  that  of  the  barometer,  also  corrected,  is  the  value 


74  QUANTITATIVE  EXERCISES 

required.  But  when  another  liquid  is  used  the  length  of  an 
equivalent  column  of  mercury  must  first  be  found.  To  do 
this,  the  height  of  the  column  is  multiplied  by  the  specific 
gravity  of  the  liquid  and  the  product  divided  by  13.596,  the 
density  of  mercury  at  0°.  If  the  liquid  is  water,  the  specific 
.gravity  corresponding  to  its  temperature  will  be  found  in  any 
collection  of  chemical  tables  ;  otherwise  it  will  probably  have  to 
be  determined. 

The  commonly  accepted  standard  for  pressure  is  that  which 
at  0°  will  support  a  column  of  mercury  760  mm.  in  height,  and 
all  gases  measured  under  other  pressures  are  corrected  to  the 
volumes  which  they  would  have  under  this.  The  formula  for 
the  correction  is 

F/= 


F,  is  the  volume  under  standard  pressure, 

F  the  observed  volume,  and 

h  the  pressure  —  with  all  necessary  corrections  applied  — 
under  which  the  gas  was  measured. 

Ordinarily  h  is  simply  the  difference  between  the  height  of 
the  barometer  and  that  of  the  mercury  column  over  which  the 
gas  was  measured  —  both  corrected  for  temperature;  but  in 
more  refined  work,  such  as  the  determination  of  atomic  weights 
and  the  densities  of  gases,  a  correction  for  latitude  and  altitude 
is  also  to  be  made.  This  is  to  be  applied,  of  course,  not  to  the 
barometer  and  the  column  separately,  but  only  to  the  difference 
between  them,  i.e.  to  h. 

II.  The  Correction  for  Temperature 

If  the  temperature  of  a  gas  is  raised,  without  change  of  pres- 
sure, from  0°  to  100°,  its  initial  and  final  volumes  are  related  to 
each  other  as  1  to  1.367.  For  each  increase  in  temperature  of 

one  degree,  the  expansion  amounts  to  0.00367,  7^0  °^  ^s  volume 


Till;   CALIBRATION   OF   EUDIOMETERS  75 

at  0°.     This  law  was  discovered  simultaneously  by  Gay-Lussac 
and  Dalton. 

The  standard  temperature  for  the  measurement  of  gases  is  0°, 
and  the  formula  for  finding  what  volume  any  gas,  measured  at  a 
higher  temperature,  would  have  at  the  standard  temperature  is 

jr 

,  in  which 


1  +  0.00367* 

V  is  the  observed  volume, 

t  the  temperature  of  the  gas  at  the  time  of  measurement,  and 
0.00367  the  expansion  coefficient  of  gases. 
If  the  temperature  at  the  time  of  measurement  is  below  0°, 
the  formula  becomes 


1-0.00367** 

The  formula  for  finding  the  volume  of  gases  under  standard 
conditions,  both  of  temperature  and  pressure,  is 


0.00367*      760* 


III.  The  Correction  for  Water  Vapor 

Gases  are  measured  either  dry  or  fully  saturated  with  water 
vapor  —  usually  in  the  latter  condition.  The  correction  of  the 
volume  of  a  saturated  gas  for  the  water  vapor  which  it  contains 
is  effected  by  subtracting  the  known  tension  or  pressure  of  the 
vapor  —  expressed  in  millimeters  of  mercury  —  from  A,  the  pres- 
sure under  which  the  gas  was  measured. 

The  complete  formula  for  the  reduction  of  observed  gas  vol- 
umes to  standard  conditions  of  temperature,  pressure,  and 

dry  ness  is 

V  h  —  tension  of  water  vapor 

1  +  0.00367*  >  760 

A  gas  standing  over  water  for  any  length  of  time  necessarily 
becomes  saturated  with  vapor.  The  same  is  true  of  a  gas  which 


76  QUANTITATIVE  EXERCISES 

is  passed  through  water,  but  it  is  to  be  remembered  with  refer- 
ence to  the  latter  method  of  effecting  saturation  that  if  the  tem- 
perature of  the  gas  should  afterwards  rise,  more  water  will  be 
required.  When  a  gas  is  collected  over  mercury,  the  water 
necessary  for  its  saturation  may  be  provided  by  moistening  the 
inner  wall  of  the  containing  vessel  before  filling  with  mercury, 
or  by  introducing  afterwards  a  small  drop  of  water. 

It  is  also  to  be  remembered  that  the  tension  of  water  vapor 
over  an  aqueous  solution  is,  at  a  given  temperature,  always  less 
than  over  pure  water.  It  is  therefore  necessary  as  a  rule, 
since  the  vapor  tension  of  but  few  solutions  is  known,  finally 
to  measure  a  gas  over  water  or  mercury,  or  in  the  dry  condition 
over  a  liquid  which,  like  concentrated  sulphuric  acid,  has  prac- 
tically no  vapor  tension  of  any  kind. 

The  student  should  practice  the  reduction  of  gas  volumes, 
applying  all  possible  corrections,  until  he  is  thoroughly  familiar 
with  the  principles  on  which  these  corrections  are  based,  and 
until  he  has  acquired  some  facility  in  computations  of  this 
kind.  Afterwards  tables  which  have  been  prepared  to  lighten 
the  labor  of  such  calculations  may  be  resorted  to.  All  required 
tables  will  be  found  in  the  collection  of  Landolt  and  Boern- 
stein.  Those  most  frequently  used  in  connection  with  gaso- 
metric  work  are  : 

Corrections  of  the  barometer  for  temperature,  for  latitude, 
and  for  altitude. 

Corrections  for  meniscus  in  tubes  of  different  diameters. 

Reduction  of  water  pressure  to  mercury  pressure. 

Values  of        . 


Values  of  1  +  0.00367  t. 

Tension  of  aqueous  vapor  over  water  and  certain  solutions. 

The  weights  of  unit  volumes  of  gases. 

Absorption  coefficients  of  gases  in  liquids. 


THE   CALIBRATION  OF  EUDIOMETERS  77 


The  Passage  of  G-ases  through  Rubber  * 

It  is  important  for  the  student  to  know  that  rubber,  which  in 
the  form  of  tubing  is  constantly  used  in  the  laboratory  to  con- 
nect the  different  parts  of  apparatus  and  to  direct  the  flow  of 
gases,  is  permeable  to  gases  —  including  water  vapor  —  and  to 
some  of  them  to  a  high  degree.  The  passage  of  gases  through 
rubber  does  not  obey  the  law  of  diffusion  which  requires  that 
the  rates  of  diffusion  shall  be  inversely  proportional  to  the 
square  roots  of  the  densities  of  the  gases.  This  will  appear 
from  the  following  table  which  gives  the  relative  volumes  of 
some  of  the  commoner  gases  which  will  pass  through  a  rubber 
septum  in  a  unit  of  time. 

GAS  KATE  OF  DIFFUSION  SQUARE  ROOT  OF  DENSITY 

Nitrogen 1.000 3.7416 

Carbonic  oxide  .     .     .-.  1.113 3.7416 

Atmospheric  air     ...  1.149 3.7947 

Marsh  gas 2.148 2.8284 

Oxygen 2.556 4.0000 

Hydrogen 5.500 1.0000 

Carbon  dioxide ....  13.585 4.5825 

According  to  the  law  of  gaseous  diffusion,  carbon  dioxide 
should  pass  through  a  septum  only  0.22  as  rapidly  as  hydrogen, 
while  its  actual  rate  of  passage  through  rubber  is  13.585  times 
as  rapid  as  that  of  nitrogen  and  2.47  times  as  rapid  as  that  of 
hydrogen.  In  other  words,  carbon  dioxide  penetrates  rubber 
16.6  times  too  rapidly  when  compared  with  nitrogen  and  11.23 
times  too  rapidly  when  compared  with  hydrogen.  Sulphur 
dioxide  exhibits  this  power  of  penetrating  rubber  to  an  even 
more  remarkable  degree  than  carbonic  anhydride.  The  disa- 
greeable odor  of  rubber  tubes  through  which  illuminating  gas 
is  passing  and  the  well-known  fact  that  the  gas  in  passing 

*  The  student  should  read  Thomas  Graham,  "  On  the  Absorption  and  Dialytic 
Separation  of  Gases  by  Colloid  Septa,"  Philosophical  Transactions,  1866,  p.  399; 
or  Researches  of  Thomas  Graham,  p.  235. 


78  QUANTITATIVE  EXERCISES 

through  such  tubes  suffers  a  notable  loss  in  illuminating  power 
are  familiar  proofs  of  the  readiness  with  which  rubber  is  pene- 
trated by  gases. 

Equality  of  gaseous  pressure  upon  the  two  sides  does  not 
prevent  the  passage  of  gases  through  an  intervening  wall  of 
rubber;  hence  gases  conducted  through  rubber  tubing  or  in 
contact  with  rubber  connections  or  stoppers  must,  in  general, 
suffer  some  change  in  composition.  The  important  practical 
rule  to  be  deduced  from  the  foregoing  statements  is  that,  in 
dealing  with  gases,  rubber  connections  and  'rubber  stoppers  are 
to  be  avoided  wherever  it  is  practicable  to  dispense  with  them. 
If  the  parts  of  an  apparatus  must  be  connected  by  means  of 
rubber,  the  ends  to  be  joined  should  be  accurately  fitted  to  each 
other  and  brought  close  together  in  the  connection.  Again,  if 
a  gas  is  to  be  transported  for  a  considerable  distance,  glass  tubes, 
with  a  minimum  of  rubber  connections,  should  be  em- 
ployed. The  common  practice  of  using  long  stretches 
of  rubber  tubing  for  this  purpose  is  a  sure  evidence 
either  of  ignorance  or  of  a  certain  lack  of  aptitude  for 
good  experimental  work. 

Figures  12  and  13  exhibit  arrangements  which  may 
FIG  12    °ften  ke   used  with  advantage    to   prevent   diffusion 
of  gases  through  rubber  connections.     The   outer  or 
"  jacket "  tubes  are  filled  with  mercury.     Such  protection,  how- 
ever, does  not  wholly  remove  the  objection  to  the  use  of  rubber 
connections,  since  rubber  is  capable  of  ab- 
sorbing and  retaining  considerable  volumes 
of  certain  gases.    This  is  especially  true  of  = 
oxygen,   carbon  dioxide,   sulphur    dioxide, 
and  some  of  the  hydrocarbons. 

Rubber  connecting  tubes  should  usually  be  tied,  especially 
when  the  gaseous  pressure  upon  the  inner  and  outer  walls  is 
unequal.  The  best  material  for  such  ligatures  is  waxed  shoe- 
maker's thread  —  so-called  "  waxed  end." 


CHAPTER  IV 

CALIBRATION    AND    GRADUATION    OF    APPARATUS   FOR    THE 
MEASUREMENT   OF   LIQUIDS 


EXERCISE  VIII 
I.    DETERMINATION  OF  THE  CAPACITY  OF  A  MEASURING  FLASK 

BY   WEIGHING  WATER 

If  a  large  and  sufficiently  accurate  balance  is  at  the  disposal 
of  the  student,  a  liter  flask  should  be  employed  in  this  exercise, 
otherwise  a  smaller  one  may  be  selected. 

Place  near  the  balance,  several  hours  before  it  is  required  for 
use,  a  sufficient  quantity  of  distilled  water.  The  containing 
vessel  should  be  closed,  since  the  temperature  of  evaporating 
water  is  usually  somewhat  below  that  of  the  surrounding 
atmosphere. 

Put  upon  the  left-hand  pan  of  the  balance  a  weight  —  a 
beaker  containing  shot  will  suffice — which  is  heavier  than  both 
the  flask  and  the  water  to  be  weighed.  Suspend  the  closed 
flask  from  the  right-hand  stirrup  by  means  of  a  platinum  wire 
and  add  weights  to  the  pan  underneath  until  equilibrium  is 
obtained,  or  until  the  deficit  or  excess  of  weight  can  be  deduced 
from  the  sensibility  of  the  balance. 

Take  the  temperature  of  the  water  and  that  of  the  air  in  the 
vicinity.  If  both  are  the  same,  fill  the  flask  nearly  to  the  mark 
on  the  neck  through  a  funnel  with  a  long  stem,  or  one  whose 
stem  has  been  extended  by  attaching  to  it  a  small  glass  tube, 
taking  care  not  to  wet  the  glass  above  the  mark.  Remove  the 
funnel  and  continue  the  filling  with  a  glass  tube  —  one  end  of 
which  is  drawn  out  to  a  fine  point  —  until  the  bottom  of  the 

79 


80 


QUANTITATIVE  EXERCISES 


meniscus  is  on  a  level  with  the  mark.  If,  in  spite  of  precau- 
tions, the  neck  above  the  mark  has  been  wet,  the  water  must 
be  removed.  This  is  best  done  by  means  of  a  strip  of  filter 
paper  rolled  into  the  form  of  a  small  cylinder. 

Place  the  filled  flask  (closed)  on  the  pan  and  add  weights 
to  equilibrium.  The  difference  between  the  weights  added 
when  the  flask  was  empty  and  afterwards  when  it  was  filled  is 
the  apparent  weight  of  the  water,  i.e.  its  weight  without  correc- 
tion for  air  displacement.  Find  its  weight  in  a  vacuum. 

Divide  the  corrected  weight  of  the  water  by  its  density. 
This  will  give  its  volume  in  cubic  centimeters  and  the  capacity 
of  the  flask  at  the  temperature  at  the  time  of  weighing.  Find 
the  capacity  of  the  flask  at  4°,  15°,  17.5°,  and  20°,  using 
0.000025  as  the  cubical  expansion  coefficient  of  glass.  The 
following  table  gives  the  density  of  water  —  the  weight  of  one 
cubic  centimeter  and  the  volume  of  one  gram  of  water  (corrected 
weight)— between  0°  and  25°. 

Density  and  Volume  of  Water  between  0°  and  25° 


TEMPER- 
ATURE 

DEKSITY 

VOLUME 
cc. 

TEMPER- 
ATURE 

DENSITY 

VOLUME 
cc. 

0° 

0.999878 

1.000122 

13° 

0.999430 

1.000570 

1 

0.999933 

1.000067 

14 

0.999297 

1.000703 

2 

0.999972 

1.000028 

15 

0.999154 

1.000847 

~3 

0.999993 

1.000007 

16 

0.999004 

1.000997 

4 

1.000000 

1.000000 

17 

0.998839 

1.001162 

5 

0.999992 

1.000008 

18 

0.998663 

1.001339 

6 

0.999969 

1.000031 

19 

0.998475 

1.001527 

7 

0.999933 

1.000067 

20 

0.998272 

1.001731 

8 

0.999882 

1.000118 

21 

0.998065 

1.001939 

9 

0.999819 

1-000181 

22 

0.997849 

1.002156 

10 

0.999739 

1.000261 

23 

0.997623 

1.002389 

11 

0.999650 

1.000350 

24 

0.997386 

1.002621 

12 

0.999544 

1.000456 

25 

0.997140 

1.002868 

APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          81 


II.   THE  GRADUATION  OF  A  MEASURING  FLASK 

Warm  one  side  of  a  piece  of  paraffin  or  beeswax  and  with  it 
rub  the  neck  of  the  flask  to  be  graduated ;  then  warm  the  neck 
over  a  flame  and  turn  the  flask  until  the  molten  wax  distributes 
itself  uniformly  over  the  glass  in  a  film  thin  enough  to  be  mod- 
erately translucent  when  cold. 

The  second  step  is  to  ascertain  how  much  water  of  the  tem- 
perature of  the  balance  room  must  be  weighed  into  the  flask  in 
order  that  the  flask  may  have  the  desired  capacity,  below  the 
mark,  at  some  standard  temperature  which,  of  course,  should  be 
the  prevailing  temperature  of  the  laboratory.  We  will  suppose 
that  the  temperature  of  the  balance  room  and  of  the  water  is 
18°,  while  the  flask  is  to  be  graduated  to  hold  a  liter  at  20°, 
the  latter  being  the  usual  temperature  of  the  laboratory.  If  the 
capacity  of  a  flask  at  20°  is  1000  cc.,  its  capacity  at  any  lower 

temperature  will  be AAAAOC    '   At  18°  it  will  be 


1  +  0.000025  t  1.000050, 

or  999.95  cc.  The  volume  of  a  gram  of  water  at  18°  (see  table) 
is  1.001339  cc. ;  the  weight  of  water  to  be  introduced  into  the 

999  95 

flask  is,  therefore,  '       ,  or  998.613  grams.    This,  however, 

1.001339 

is  weight  in  a  vacuum.  To  find  the  weight  of  the  water  in 
the  air,  we  must  deduct  the  weight  of  999.95  cc.  of  air,  less  the 
weight  of  the  air  displaced  by  the  brass  weights.  The  volume 

998  613 
of  the  weights  is  — — '— — ,  or  118.88  cc. ;  and  the  volume  of  the 

air  whose  weight  is  to  be  deducted  is  999.95  — 118.88,  or 
881.07  cc.  The  average  weight  of  a  cubic  centimeter  of  air  is 
1.2  milligrams;  881.07  X  0.0012,  or  1.057  grams,  is  therefore 
the  weight  to  be  deducted.  That  is,  in  order  that  the  flask 
may  have  a  capacity  of  one  liter  at  20°,  there  must  be  weighed 
into  it  at  18°  998.613  - 1.057,  or  997.556  grams  of  water. 


82  QUANTITATIVE  EXERCISES 

The  general  formula  for  finding  the  weight  in  the  air  of  an 
object  whose  weight  in  a  vacuum  is  known  is 

W 

,  in  which 


JFis  the  weight  in  a  vacuum, 

d  the  density  of  the  object,  and 

d,  the  density  of  the  weights. 

Having  ascertained  the  weight  of  water  required  and  having 
weighed  the  flask,  fill  in  through  a  funnel  having  a  long  stem 
until  the  water  reaches  the  neck  and  then  weigh.  Calculate 
approximately  what  volume  of  water  remains  to  be  introduced  and 
add  very  nearly  the  required  amount  from  a  graduated  pipette. 
Weigh  again,  remove  weights  equal  to  the  weight  of  the  water 
still  to  be  introduced,  and  then  add  water  from  a  small  glass 
tube  with  a  fine  delivering  end  until  equilibrium  is  obtained.  In 
view  of  the  volume  of  water  required  to  make  an  appreciable 
change  in  the  level  of  the  meniscus,  it  is  probably  useless,  in 
graduating  a  measuring  flask,  to  attempt  to  weigh  the  water 
accurately  to  within  less  than  10  milligrams,  since  this  weight 
of  water  would  have  a  volume  of  only  about  0.01  cc. 

With  a  sharp-pointed  piece  of  steel,  e.g.  a  round  file  ground 
to  a  sharp  point,  scratch  a  number  of  short  horizontal  lines  in 
the  wax  on  a  level  with  the  bottom  of  the  water  meniscus. 
Continue  the  line  entirely  around  the  neck  and  etch  it  into  the 
glass  with  hydrofluoric  acid.  In  some  laboratories  there  is  in 
use  a  simple  adjustable  arrangement  which  holds  the  steel  point 
in  the  proper  position  against  the  paraffined  neck  while  the 
flask  is  revolved.  Lines  cut  in  this  way  are  usually  more  satis- 
factory than  those  produced  in  the  manner  described  above. 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          83 

III.    THE  CALIBRATION  OF  BURETTES  BY  MEANS  OF  MERCURY 

A  burette  may  be  calibrated  with  mercury  in  much  the  same 
manner  as  a  eudiometer;  but,  since  these  instruments  are  used 
mainly  for  the  measurement  of  liquids  which  leave  a  film  upon 
the  glass,  and  hence  deliver  less  than  their  full  capacity,  the 
volume  of  the  film  must  be  determined  and  applied  as  a  correc- 
tion to  the  results  obtained  by  the  mercury.  The  method  is 
sufficiently  accurate,  but  so  cumbersome  that  it  is  not  often 
resorted  to.  The  exercise  may  therefore  be  omitted.  Never- 
theless the  student  should  construct  for  himself  a  complete 
working  plan  for  such  a  calibration. 

The  System  of  Mohr 

Owing  to  the  somewhat  laborious  corrections  which  are  ren- 
dered necessary  by  a  strict  adherence  to  the  liter  as  the  unit  of 
volume,  Mohr  proposed  to  regard  as  the  standard  for  liquid- 
volumetric  work  the  space  occupied  by  a  kilogram  of  water 
when  the  same  is  weighed  in  the  air  with  brass  weights  at  a 
temperature  of  17.5°.  The  advantages  claimed  by  the  author 
for  his  system  are : 

1.  The  correctness  of  graduated  apparatus  is  easily  tested. 

2.  The  measuring  apparatus  can  be  used  without  correction 
—  at  a  temperature  easily  secured   and   maintained  —  for  the 
determination  of  the  specific  gravities  of  liquids. 

The  system  of  Mohr  obtained  a  wide  currency  in  several 
countries,  though  modified  here  and  there  by  the  substitution  of 
other  temperatures,  e.g.  15°  for  the  standard  17.5°  of  its  author. 
Of  late,  however,  the  system  has  rapidly  lost  ground  and  it  is 
doubtless  destined  to  disappear  altogether  in  the  near  future. 

The  following  table  gives  for  several  temperatures  the  cor- 
rected weight  and  the  volume  of  the  water  which  in  the  air  will 
balance  a  thousand  grams  of  brass  of  specific  gravity  8.4,  i.e.  a 
kilogram  brass  weight. 


84 


QUANTITATIVE  EXERCISES 


TEMPEBATURE 

COBKECT  WEIGHT 

VOLUME 

0° 

1001.0573  gr. 

1001.1851  cc. 

4 

1001.0511 

1001.0571 

10 

1001.0575 

1001.3218 

12.5 

1001.0578 

1001.5811 

15 

1001.0582 

1001.9168 

17.5 

1001.0587 

1002.3301 

20 

1001.0592 

1002.8122 

22.5 

1001.0599 

1003.3626 

25 

1001.0606 

1003.9742 

27.5 

1001.0614 

1004.6505 

30 

1001.0623 

1005.3804 

It  will  be  seen  that  the  strictest  adherence  to  a  fixed  temper- 
ature is  necessary  in  order  that  a  volume  unit  like  that  proposed 
by  Mohr  may  have  real  value  as  a  working  standard.  But  the 
temperature  recommended  by  the  author  of  the  system,  17.5°, 
has  not  been  adhered  to  in  the  graduation  of  apparatus ;  neither 
has  any  other  particular  standard  temperature  been  adopted  by 
common  consent.  This  divergence  —  due  apparently  to  the  lack 
of  uniformity  among  the  different  laboratories  in  respect  to 
what  may  be  called  prevailing  temperature  —  has  resulted  in 
much  confusion  and,  consequently,  in  a  lack  of  precision  in  volu- 
metric work.  More  unfortunate  still  has  been  the  confusion  of 
mind  and  the  errors  in  practice  due  to  the  custom  of  calling  the 
Mohr  volume  unit  a  liter  and  its  submultiples  cubic  centimeters. 
It  may  be  asserted  with  considerable  confidence  that  the  intro- 
duction of  the  Mohr  system  is  largely  responsible  for  the  wide- 
spread impression  that  the  volumetric  system  of  analysis  is 
necessarily  less  accurate  than  the  gravimetric,  and  that  it  is 
therefore  suited  only  to  work  of  a  rough  kind.  A  sufficient 
explanation  of  the  origin  of  this  mistaken  judgment  appears 
when  one  finds  in  use  side  by  side  and  uncalibrated,  as  he  often 
may,  apparatus  which  has  been  graduated  according  to  the  Mohr 
system  for  17.5°  and  for  15°,  and  also  according  to  the  true  liter. 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS 


85 


EXERCISE  IX 

THE  CALIBRATION  AND  GRADUATION  OF  MEASURING 
FLASKS  AND  THE  CALIBRATION  OF  BURETTES 

* 

With  the  aid  of  the  apparatus  which  is  explained  below,  the 
student  should  calibrate  —  and  regraduate  if  they  are  found  to 
be  incorrect  —  the  fol- 
lowing flasks :  1  1.,  J  1., 
and  ^  1.  He  should  also 


calibrate   not  less 
three  burettes. 


than 


.200 


The  apparatus  repre- 
sented in  Fig.  14  is 
employed  both  for  the 
graduation  and  the  cali- 
bration of  half-liter  and 
liter  measuring  flasks. 
The  delivering  capacity 
of  the  bulb  between  the 
mark  on  the  upper  stem 
and  the  zero  mark  on 
the  lower  one  must  not 
exceed  500  cc.  at  the 
highest  temperature  at 
which  the  instrument  is 
to  be  used;  while  the 
combined  delivering 
capacity  of  the  bulb  and 
the  graduated  portion  of 
the  lower  stem  must  not 
be  less  than  half  a  liter 
at  0°.  The  lower  stem 
is  graduated  in  millimeter  divisions  which  are  numbered  in  both 
directions  in  order  that  the  instrument  may  be  used  in  the  in- 
verted position  if  desired, 


FIG.  15 


86  QUANTITATIVE  EXERCISES 

For  the  purpose  of  explaining  the  method  of  preparing  an 
instrument  of  this  kind  for  use,  an  account  of  an  actual  calibra- 
tion will  be  given.  The  water  delivered  by  the  bulb  and  also 
that  delivered  by  the  graduated  portion  of  the  lower  stem  was 
weighed  and  its  temperature  noted.  The  delivering  capacity  of 
the  bulb  and  of  the  stem  was  then  calculated  by  the  formula 

V=P^  in  which 
a 

P  is  the  weight  of  the  water  when  weighed  in  the  air  with 
brass  weights  (497.769  grams  for  the  bulb  and  3.0708  grams 
for  the  stem), 

p  is  the  weight  in  a  vacuum  of  one  gram  of  water  weighed  in 
the  air  with  brass  weights  (1.001059  grams  at  ordinary  tempera- 
tures), and 

d  is  the  density  of  the  water  at  the  temperature  of  weighing 
(21.4°). 

In  this  way  the  delivering  capacity  of  the  bulb  at  21.4°  was 
found  to  be  499.324  cc.,  and  that  of  the  stem  at  the  same  tem- 
perature 3.081  cc. 

The  delivering  capacity  of  the  bulb  at  0°,  4°,  10°,  12.5°,  15°, 
17.5°,  20°,  22.5°,  25°,  27.5°,  and  30°  was  calculated  by  the 
formula  * 

V,  =  V  (1  +  7  (t,  -  *)),  in  which 

7  is  the  cubical  expansion  coefficient  of  glass  (0.000025),  t f  is 
the  temperature  0°,  4°,  or  10°,  etc.,  and  t  is  the  temperature  of 
the  water  at  the  time  of  weighing. 

The  results  of  these  calculations  are  given  in  the  second 
column  of  the  table  which  follows.  No  corresponding  calcula- 
tion was  made  for  the  stem  because  the  change  in  its  capacity 
between  0°  and  30°  amounts  to  less  than  0.0025  cc.  The 
inequalities  of  its  bore  were  also  found  to  be  insignificant.  Each 
millimeter  division  of  the  stem  was  therefore  regarded  as  having 
a  delivering  capacity  of  0.03081  cc.  at  all  temperatures  between 
0°  and  30°, 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          87 

The  instrument  was  then  ready  for  rinding  the  capacity  of  any 
graduated  half-liter  or  liter  flask  at  any  of  the  temperatures 
mentioned  above.  For  this  purpose  a  small  glass  tube  drawn 
out  and  curved  at  one  end,  and  long  enough  to  reach  the  mark 
on  the  neck  of  the  measuring  flask,  is  attached  to  the  delivering 
tube  of  the  three-way  stopcock,  while  the  other  tube  of  the  stop- 
cock is  connected  with  a  supply  of  water  situated  somewhat 
higher  than  the  pipette.  Water  is  allowed  to  flow  in  and  fill 
the  whole  apparatus  —  including  the  delivery  tube  below  the 
stopcock  —  to  the  mark  on  the  upper  stem.  The  flask  whose 
capacity  it  is  desired  to  find  is  filled  to  the  mark  from  the 
pipette.  If  the  quantity  delivered  from  the  stem  is  added  to 
any  number  in  the  second  column  of  the  table,  we  shall  obtain 
the  capacity  of  the  flask  at  the  corresponding  temperature  in  the 
first  column.  This  is  true  of  a  half-liter  flask ;  in  the  case  of  a 
liter  flask  the  pipette  must  be  twice  filled,  and  the  volume 
delivered  by  the  stem  is,  of  course,  to  be  added  to  twice  the 
numbers  in  the  second  column. 

Since  the  pipette  and  the  flask  are  of  the  same  material,  and 
therefore  expand  and  contract  alike  for  equal  changes  of  tem- 
perature, the  temperature  of  the  water  at  the  time  of  the  experi- 
ment need  not  be  known. 

The  next  step  was  to  find  to  what  points  on  the  graduated 
stem  the  water  must  be  drawn  in  order  to  graduate  a  half-liter 
flask  for  each  of  the  temperatures  included  in  the  first  column. 
This  was  done  by  dividing  the  difference  between  500  and  the 
several  numbers  in  the  second  column  by  0.03081,  the  deliver- 
ing capacity  of  one  division  of  the  stem.  The  results  are  given 
in  the  third  column.  To  graduate  a  half-liter  flask  for  any  of 
the  temperatures  in  the  first  column,  it  is  necessary  only  to 
empty  into  it  the  bulb  and  then  the  corresponding  number  of 
stem  divisions  indicated  in  the  third  column.  If  it  is  desired 
to  have  the  flask  correct  at  0°,  30.9  stem  divisions  will  be 
added;  at  4°,  29.3  divisions,  etc.  Here  again  the  result  is 
correct  whatever  may  be  the  temperature  of  the  water  at  the 


88 


QUANTITATIVE  EXERCISES 


time  of  the  experiment.  To  graduate  a  liter  flask,  the  bulb 
twice  full  and  twice  the  indicated  number  of  stem  divisions  must 
be  added. 

To  prepare  the  instrument  for  use  in  the  graduation  and  veri- 
fication of  apparatus  when  the  so-called  Mohr  system  is  to  be 
employed  (i.e.  when  the  correction  for  air  displacement  is  to 
be  dispensed  with),  the  quantities  in  the  second  column  were 
divided  by  the  volumes  at  the  different  temperatures  of  one 
gram  of  water  when  weighed  in  the  air  with  brass  weights. 
These  are : 


TEMPERATURE 

VOLUME 

TEMPERATURE 

VOLUME 

At  0° 

1.001185  CC. 

At  20° 

1.002812  CC. 

4 

1.001057 

22.5 

1.003363 

10 

1.001322 

25 

1.003974 

12.5 

1.001581 

27.5 

1.004651 

15 

1.001917 

30 

1.005380 

17.5 

1.002330 

The  results  are  recorded  in  the  fourth  column  of  the  table. 
The  final  step  was  to  find  to  what  point  on  the  stem  the  water 
must  be  drawn  in  order  to  graduate  a  half-liter  flask  on  the 
Mohr  system  for  each  of  the  temperatures  recorded  in  the  first 
column.  For  this  purpose  the  capacity  of  the  stem  (3.081  cc.) 
was  divided  by  1.00233,  the  true  volume  of  the  Mohr  unit  at 
17.5°,  which  gave  3.0738  as  the  capacity  of  the  stem  in  Mohr 
units,  or  0.030738  as  the  capacity  of  a  single  division.  The 
differences  between  500  and  the  several  numbers  in  the  fourth 
column  were  then  divided  by  0.030738,  giving  the  numbers 
which  are  recorded  in  the  fifth  column.  Strictly,  each  differ- 
ence should  be  divided  by  a  different  number,  but  the  maximum 
error  which  could  ever  result  from  regarding  the  volume  of  a 
gram  of  air-weighed  water  as  constant  between  0°  and  30°  is  in 
this  instance  less  than  0.01  cc.  and  therefore  inappreciable. 


APPARATUS  FOR  MEASUREMENT   OF  LIQUIDS 


89 


TEMPERA- 
TURE 

CAPACITY  OF  BULB 

IN  CC. 

DIVISIONS 
OF  STEM  TO 

BE  ADDED 

CAPACITY  OF  BULB  IN 
MOHR  UNITS 

DIVISIONS 
OF  STEM  TO 

BE  ADDED 

0° 

499.048 

30.9 

498.457 

50.2 

4 

499.098 

29.3 

498.571 

46.5 

10 

499.176 

26.7 

498.517 

48.2 

12.5 

499.208 

25.7 

498.420 

51.4 

15 

499.241 

24.6 

498.286 

56.1 

17.5 

499.273 

23.6 

498.112 

61.4 

20 

499.304 

22.6 

497.937 

67.1 

22.5 

499.338 

21.5 

497.664 

76.0 

25 

499.372 

20.4 

497.395 

84.7 

27.5 

499.403 

19.4 

497.091 

94.6 

30 

499.436 

18.3 

497.763 

105.3 

Capacity  of  one  stem  division  in  cc.  =  0.03081. 

Capacity  of  one  stem  division  in  Mohr  units  =  0.030738. 

The  apparatus  represented  in  Fig.  15  is  used  for  the  calibra- 
tion and  graduation  of  smaller  flasks.  The  capacities  of  the 
bulbs  and  of  the  graduated  stems  are  subject  to  the  same  limita- 
tions as  in  the  preceding  case ;  that  is,  the  delivering  capacity 
of  the  smaller  bulb  must  not  exceed  50  cc.  and  that  of  the  larger 
one  200  cc.  at  the  highest  temperature  at  which  the  instrument 
is  to  be  used,  while  the  combined  capacity  of  each  bulb  and  its 
graduated  stem  must  not  be  less  than  50  cc.  and  200  cc.  respec- 
tively at  0°.  The  delivering  capacity  of  each  bulb  and  stem  at 
different  temperatures  is  determined  and  the  results  are  tabu- 
lated in  the  manner  previously  explained.  50-cc.,  200-cc.,  and 
250-cc.  flasks  can  be  graduated  or  their  capacities  determined 
by  a  single  rilling  of  the  pipette.  To  graduate  or  calibrate  a 
100-cc.  flask,  the  smaller  bulb  must  be  twice  filled.  The  stop- 
cock may,  of  course,  be  attached  to  either  end  of  the  instrument. 

The  apparatus  represented  in  Fig.  16  is  for  the  calibration 
of  burettes.  The  graduation  upon  the  stem  is  in  millimeters. 
The  largest  bulb  has  a  delivering  capacity  of  somewhat  less  than 


90 


QUANTITATIVE  EXERCISES 


50  cc.  and,  together  with  the  graduated  stem  above,  is  employed 
to  determine  the  total  delivering  capacity  of  a  burette  at  any 
temperature.  The  limitations  in  respect  to  capacity  are  here 

the  same  as  in  the  case  of  the  pipettes 
previously  described ;  that  is,  the  bulb 
must  deliver  less  than  50  cc.  at  30°, 
while  the  bulb  and  stem  together  must 
deliver  not  less  than  50  cc.  at  0°.  The 
delivering  capacity  of  the  stem  and  the 
bulb  at  different  temperatures  is  deter- 
mined in  the  same  way  as  in  the  case 
of  the  pipettes,  and  the  results  are 
tabulated  in  the  same  form. 

The  smaller  bulbs  are  employed  to 
determine  the  inequalities  of  caliber  in 
a  burette,  The  smaller  one  delivers 
less  than  2  cc.  at  30°,  while  the  bulb 
and  the  graduated  portion  of  the  stem 
underneath  together  deliver  not  less 
than  2  cc.  at  0°.  The  larger  of  the  two 
bulbs  and  the  graduated  portion  of  the 
stem  above  it  are  subject  to  similar 
limitations.  As  will  appear  when  the 
method  of  using  the  apparatus  is  ex- 
plained, the  capacity  of  the  small  bulbs 
need  not  be  determined  by  weighing 
the  water  which  they  deliver. 

Between  the  pipette  and  the  burette 
a  T  tube  is  inserted  whose  third  limb 
is  connected  with  a  supply  of  water 
situated  above  the  upper  limit  of  the 
graduation  on  the  burette.  By  means  of  this  arrangement  the 
burette  and  the  pipette  may  be  filled  independently. 

The  procedure  in  calibrating  a  burette  is  as  follows :   The 
burette  and  the  pipette  are  both  filled  with  water  to  the  upper 


FIG.  16 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          91 

limits  of  their  graduation.  The  pipette  is  then  emptied  to  the 
beginning  of  the  graduation  under  the  large  bulk.  Next,  the 
water  in  the  burette  is  allowed  to  flow  into  the  pipette  until 
the  whole  of  the  graduated  portion  is  emptied.  The  water  rises 
into  the  stem  of  the  pipette,  and  the  total  delivering  capacity  of 
the  burette  at  any  temperature  may  be  found  by  adding  to  the 
known  delivering  capacity  of  the  bulb  at  that  temperature  the 
volume  of  the  water  in  the  stem. 

Having  found  its  total  capacity,  the  burette  is  again  filled, 
and  the  water  in  the  pipette  is  drawn  down  to  the  zero  mark 
under  the  smallest  bulb.  The  bulb  is  then  filled  from  the 
burette  and  the  reading  on  the  burette  recorded.  The  bulb  is 
emptied,  refilled,  and  a  second  reading  recorded.  This  series  of 
operations  is  repeated  until  less  than  a  bulbful  of  water  remains 
in  the  graduated  portion  of  the  burette.  The  next  step  is  to 
find  the  volume  of  the  water  drawn  off  in  the  manner  described, 
and  through  this  the  delivering  capacity  of  the  bulb.  How  this 
is  to  be  accomplished  will  be  explained  by  means  of  the  follow- 
ing example  of  a  calibration. 

A  burette  which  had  been  graduated  according  to  the  Mohr 
sytem  for  15°,  but  which  was  to  be  used  for  solutions  prepared 
for  20°, — that  being  the  prevailing  temperature  of  the  laboratory, 
—  was  found  to  have  at  20°  a  delivering  capacity  of  50.258  cc. 
The  burette  was  filled  and  the  water  drawn  off  in  quantities 
which  exactly  filled  the  smallest  bulb  of  the  pipette.  The  read- 
ings upon  the  burette  corresponding  to  the  successive  bulbfuls 
are  given  in  the  table  under  R.  The  twenty-fifth  and  last  read- 
ing was  49.52.  The  capacity  of  the  burette  to  this  point  on  its 
graduation  was  found,  by  the  proportion 

50  :  50.258  : :  49.52  :  z,  to  be  49.7755  cc. 
The  capacity  of  the  bulb  used  in  the  calibration  was,  therefore, 

49  7755 
— ,  or  1.99102  cc.    The  true  volumes  corresponding  to  the 

2b 

readings  were  then  found  by  multiplying  1.99102  by  the  series 
of  numbers  from  1  to  25.  The  results  are  given  in  the  table 


92 


QUANTITATIVE  EXERCISES 


under  C.R.  The  differences  between  the  corresponding  numbers 
under  R.  and  C.R.,  i.e.  the  quantities  to  be  added  as  corrections 
to  the  actual  readings,  are  given  under  D.  Finally,  a  curve  of 
corrections  for  the  burette  was  plotted  upon  cross-ruled  paper, 
using  a  line  representing  the  graduation  upon  the  burette  as  the 
axis  of  abscissas  and  the  corrections  under  D  as  ordinates. 


R. 

C.R. 

D. 

R. 

C.R. 

D. 

1 

1.95 

1.99 

+  0.03 

14 

27.70 

27.87 

+  0.17 

2 

3.94 

3.98 

+  0.04 

15 

29.70 

29.87 

+  0.17 

3 

5.93 

5.97 

+  0.04 

16 

31.68 

31.86 

+  0.18 

4 

7.94 

7.96 

+  0.02 

17 

33.65 

33.85 

+  0.20 

5 

9.91 

9.96 

+  0.05 

18 

35.62 

35.84 

+  0.22 

6 

11.90 

11.95 

+  0.05 

19 

37.60 

37.83 

+  0.23 

7 

13.85 

13.94 

+  0.09 

20 

39.59 

39.82 

+  0.23 

8 

15.84 

15.93 

+  0.09 

21 

41.58 

41.81 

+  0.23 

9 

17.83 

17.92 

+  0.09 

22 

43.56 

43.80 

+  0.24 

10 

19;78 

19.91 

+  0.13 

23 

45.55 

45.79 

+  0.24 

11 

21.74 

21.90 

+  0.16 

24 

47.54 

47.78 

+  0.24 

12 

23,75 

23.89 

+  0.14 

25 

49.52 

49.78 

+  0.26 

13 

25.74 

25.88 

+  0.14 

50.00 

50.26 

+  0.26 

The  system  of  graduation  and  calibration  of  volumetric  appa- 
ratus here  described  is  equally  applicable  to  all  varieties  of 
graduation  —  whether  according  to  the  Mohr  or  the  metric  unit, 
or  in  millimeters  —  and  also  to  all  temperatures.  With  its  aid 
volumetric  apparatus  of  the  most  miscellaneous  character,  as 
regards  system  and  accuracy  of  graduation,  can  be  brought  into 
harmony  and  used  side  by  side. 

All  of  the  required  apparatus  —  excepting,  of  course,  the 
three-way  stopcock  —  can  be  made  in  the  laboratory  from  ordi- 
nary full-pipettes  and  without  the  aid  of  expert  workers  in 
glass.  How  simply  this  may  be  done  in  the  case  of  the  instru- 
ments for  the  graduation  and  calibration  of  measuring  flasks 
will  appear  from  the  appended  directions  for  the  conversion  of 
a  100-cc.  pipette. 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          93 

Select  a  100-cc.  full-pipette  having  the  mark  on  the  upper 
stem  near  the  bulb.  Substitute  it  —  delivery  end  up  —  for  the 
calibrating  pipette  in  the  arrangement  represented  in  Fig.  16. 
Fill  the  burette  and  pipette  with  water  and  then  empty  the 
latter  to  the  mark  on  the  stem  nearest  the  stopcock.  Fill 
into  the  pipette  from  the  burette  about  99.5  cc.  of  water  and 
mark  the  stem  on  a  level  with  the  meniscus.  Paraffin  the  stem 
which  was  connected  with  the  stopcock,  graduate  it  in  milli- 
meters beginning  with  the  original  mark,  and  etch  the  gradua- 
tion into  the  glass.  Finally,  determine  accurately  the  capacity 
of  the  graduated  portion  of  the  stem  and  of  the  bulb  by  weigh- 
ing the  water  which  they  will  deliver.  For  some  purposes  it  is 
convenient  to  have  both  stems  of  the  pipette  graduated. 

Directions  are  also  given  for  the  conversion  of  a  full-pipette 
into  one  for  the  calibration  of  burettes. 

Blow  a  small  bulb  —  one  holding  less  than  2  cc.  —  at  a  con- 
venient distance  from  the  end  of  the  upper  stem  of  a  50-cc. 
pipette.  Make  a  mark  on  the  stem  between  the  bulb  and  the 
end,  but  quite  near  the  bulb.  Place  the  pipette  in  the  arrange- 
ment represented  in  Fig.  16,  and  fill  burette  and  pipette  with 
water.  Empty  the  pipette  to  the  mark  which  was  made  below 
the  little  bulb,  and  then  fill  into  the  pipette  from  the  burette, 
as  exactly  as  may  be,  2  cc.  of  water.  Mark  the  stem  on  a  level 
with  the  meniscus.  Fill  in,  further,  about  49.7  cc.  of  water. 
If  that  volume  of  water  fills  the  large  bulb  and  enters  the  stem 
above,  no  second  mark  between  the  two  bulbs  will  be  required, 
and  the  upper  stem  is  to  be  marked  on  a  level  with  the  menis- 
cus. If  it  does  not  enter  the  stem,  attach  a  strip  of  gummed 
paper  to  the  stem  between  the  bulbs  and  empty  the  pipette  to 
some  point  above  the  smaller  bulb.  Mark  the  place  on  the 
paper  and  again  fill  in  49.7  cc.  of  water  from  the  burette.  If 
the  given  volume  of  water  now  enters  the  upper  stem,  mark  its 
limits  on  both  stems  and  graduate  the  remainder  of  the  upper 
one  in  millimeters.  Finally,  determine  exactly  the  delivering 
capacity  of  the  bulb  and  of  the  graduated  stem. 


94  QUANTITATIVE  EXERCISES 

The  Graduation  of  Glass  Tubes 

The  simple  method  of  Bunsen  (Gasometrische  Methoden, 
2.  Auflage,  p.  28)  is  to  be  recommended  for  the  graduation  in 
millimeters  of  tubes  of  all  kinds.  There  are  required  for  the 
work  a  thick  board,  2  meters  by  about  2  decimeters,  and  a 
marking  instrument  resembling  a  beam  compass. 

The  board  is  usually  cut  transversely  into  two  pieces  of  equal 
length  which  are  hinged  together.  It  is  also  provided  with  a 
semicircular  or  V-shaped  groove,  running  from  end  to  end,  to 
receive  the  standard  graduated  tube  and  the  tube  to  be  grad- 
uated. When  instruments  having  bulbs  are  to  be  graduated, 
the  wood  may  be  cut  away  in  the  proper  places  to  receive  the 
bulbs  and  thus  allow  the  tubes  to  lie  on  the  bottom  of  the 
groove.  On  each  side  of  the  groove,  at  a  distance  of  about 
60  mm.  from  its  center  and  parallel  to  it,  is  a  row  of  six 
threaded  brass  posts.  Each  post  is  provided  with  a  thumb  nut 
and  a  strip  of  brass  (75  x  30  mm.)  which  has  been  bent  to  right 
angles  at  a  distance  of  6  mm.  from  the  ends.  There  are  also 
four  rectangular  strips  of  brass  (850  x  45  mm.)  which,  in  con- 
junction with  the  short  strips  upon  the  posts,  are  employed  to 
hold  the  tubes  in  place  in  the  groove  and  to  assist  in  guiding 
the  graduation.  One  of  the  long  edges  of  two  of  them  is 
notched  at  points  5  mm.  apart,  the  alternate  notches  being  some- 
what deeper  than  the  others. 

The  wooden  beam  of  the  compass  has  a  length  of  one  meter ; 
hence  the  curvature  of  the  short  lines  cut  by  it  is  imperceptible. 
The  point  of  the  compass  which  is  used  in  cutting  away  the 
wax  should  not  be  round  in  form  but  should  resemble,  rather, 
the  cutting  end  of  a  woodworker's  chisel. 

The  tube  to  be  graduated,  by  warming  and  turning  it  over  a 
flame,  is  carefully  covered  with  a  thin  coat  of  beeswax  or  paraf- 
fin and  laid  into  one  end  of  the  groove.  The  notched  strips 
of  brass  are  brought  up  on  either  side  until  the  space  between 
them  is  equal  to  the  length  of  the  lines  to  be  cut  and  are  then 


APPARATUS  FOR  MEASUREMENT   OF   LIQUIDS  95 

fastened  in  place  by  means  of  the  strips  and  nuts  on  the  posts. 
A  tube  with  a  correct  scale,  e.g.  a  eudiometer,  is  fastened  in  a 
similar  manner  in  the  other  end  of  the  groove.  One  of  the 
steel  points  of  the  compass  is  dropped  into  a  line  on  the  stand- 
ard scale,  and  with  the  other  a  line  is  cut  in  the  wax.  The 
point  is  then  dropped  into  the  next  groove  and  a  second  line 
cut,  and  so  on  until  the  graduation  is  completed.  As  is  usual 
in  a  millimeter  scale,  the  fifth  and  tenth  lines,  owing  to  the 
notching  of  the  edges  of  the  metallic  strips,  will  be  longer  than 
the  others,  and  the  latter  will  be  longer  than  the  former.  The 
tenth  lines  should  be  numbered,  but  in  order  that  the  reading 
of  the  numbers  may  be  easy,  the  stylus  with  which  they  are 
engraved  in  the  wax  must  not  have  a  too  fine  point.  If  the 
wax  has  been  scraped  from  the  glass  in  places  by  the  metallic 


FIG.  17 

strips,  the  injury  may  be  repaired  with  the  aid  of  a  warmed 
knife  blade  or  with  a  piece  of  hot  glass. 

The  arrangement  which  has  been  described  is  the  one  usually 
employed  in  graduating  ordinary  eudiometers  (Fig.  17).  When 
smaller  tubes  are  to  be  graduated,  they  must  be  raised  above 
the  groove  by  notched  blocks  which  should  have  a  length  equal 
to  the  width  of  the  board.  These  blocks  should  be  provided 
with  two  holes  or  slots  for  the  brass  posts.  If  tubes  attached 
to  bulbs  are  to  be  graduated,  notched  metallic  strips  which  are 
shorter  than  those  mentioned  will  probably  be  required.  In  the 
place  of  the  grooved  and  hinged  board  a  half  dozen  notched 
blocks  may  be  used.  These  should  have  a  length  of  about  2 
decimeters.  The  proper  depth  and  also  the  size  of  the  notch 
will  be  determined  by  the  form  and  size  of  the  apparatus  to  be 
graduated.  Each  block  is  provided  with  two  threaded  posts, 


96  QUANTITATIVE  EXERCISES 

one  upon  either  side  of  the  notch,  and  with  screws  by  which 
it  may  be  fastened  to  a  table  top.  To  hold  the  apparatus  in 
place  and  to  guide  the  graduation,  the  metallic  strips  previously 
described  may  be  used.  It  is  desirable  that  the  distances  between 
the  points  of  the  compass  should  be  adjustable.  Some  of  the 
recent  forms  of  the  extension-beam  trammels  used  by  mechanics 
fulfill  this  condition  most  satisfactorily. 

The  graduation  may  be  etched  into  the  glass  with  a  very  con- 
centrated solution  of  hydrofluoric  acid,  which  is  best  applied  with 
a  swab  of  cotton  wool  attached  to  the  end  of  a  wire.  If  the 
graduated  portion  of  the  instrument  is  kept  thoroughly  wet 
with  fresh  quantities  of  the  acid,  an  exposure  of  about  two 
minutes  is  required  for  the  proper  etching  of  the  scale.  A  some- 
what better  effect  is  obtained  by  using  the  acid  in  the  dry,  gas- 
eous condition.  A  suitable  bath  for  this  purpose  is  easily  made 
by  lining  a  narrow  wooden  trough  or  box  with  sheet  lead.  A 
quantity  of  ground  fluor  spar  is  spread  over  the  bottom  of  the 
trough  and  covered  with  concentrated  sulphuric  acid.  The 
tube  is  placed  on  top  and  the  part  to  be  etched  covered  with 
a  curved  piece  of  sheet  lead.  To  determine  the  length  of  time 
during  which  it  is  necessary  to  expose  the  apparatus  in  order 
to  secure  a  satisfactory  result,  it  is  well,  if  practicable,  to 
place  a  trial  tube  in  the  bath  at  the  same  time,  which  may 
be  removed  at  intervals  for  the  purpose  of  examining  the  lines 

t ^   with  the  finger  nail.     Tubes  to  be  etched  by 

the  gaseous  acid  must  be  closed  with  corks, 
and  the  whole  of  the  exposed  exterior  must 
be  covered  with  wax. 

If  it  is  required  to  divide  a  fixed  arbi- 
trary  length  of  tube  into  a  given  number 
of  equal  spaces,  as  happens  when  the  grad- 
uation of  burettes  and  similar  apparatus  is  undertaken,  Bunsen's 
arrangement  represented  in  Fig.  18a  may  be  substituted  for  the 
standard  millimeter  scale.  It  consists  of  a  system  of  converging 
lines  etched  on  a  plate  of  hard  glass.  Measured  on  the  line  ab, 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          97 

or  any  line  parallel  to  it,  the  converging  lines  are  equidistant. 
Suppose  now  it  is  required  to  divide  a  length  of  20  mm.  into 
15  equal  spaces.  A  straight  edge  would  be  placed  upon  the 
plate  parallel  to  ab  and  moved  towards  the  apex  until  the 
distance  between  the  first  and  the  fifteenth  line,  measured  on 
the  straight  edge,  is  equal  to  20  mm.,  when  it  would  be  clamped 
to  the  glass.  The  straight  edge  would  next  be  placed  in  line 
with  the  piece  to  be  marked  and  the  grad- 
uation transferred  in  the  usual  manner.  It 
is  somewhat  difficult  to  secure  and  main- 
tain the  right  relation  between  the  straight 
edge  and  the  base  line  ab ;  it  is  therefore 
better  to  employ  in  the  place  of  the  Bunsen  system  of  conver- 
ging lines  a  system  of  equidistant  parallel  lines  (Fig.  18b).  The 
intersections  will  then  be  equidistant  whatever  the  position  of 
the  straight  edge  on  the  ruled  plate. 

It  is  sometimes  necessary  to  prepare  for  a  special  purpose  a 
burette  or  other  liquid-measuring  apparatus  which  differs  in 
some  way  from  the  instruments  usually  kept  in  stock. 
As  a  rule,  the  graduation  of  such  pieces  presents  no 
great  difficulty.  We  will  suppose  that  the  bulb  tube 
represented  in  Fig.  19  is  to  be  so  graduated  that  the 
bulb,  together  with  a  short  section  of  the  upper  and 
lower  tubes,  will  deliver  100  cc.  at  20°,  while  the 
remainder  of  the  lower  tube  will  deliver  cubic  centime- 
ters at  the  same  temperature.  The  directions  for  the 
operation  would  be  as  follows :  Mark  the  point  above 
the  bulb  at  which  it  is  desired  to  have  the  graduation 
begin  and  put  the  tube  in  the  place  of  the  burette  in 
the  arrangement  represented  in  Fig.  16.  Substitute  for  the 
pipette  in  the  figure  a  100-cc.  calibrating  pipette  and  find  in 
the  table  which  goes  with  the  instrument  how  many  stem  divi- 
sions must  be  added  to  the  capacity  of  the  bulb  if  the  instru- 
ment is  to  deliver  100  cc.  at  20°.  Fill  both  instruments  with 
water  and  empty  the  pipette  to  the  zero  mark  under  the  bulb. 


93  QUANTITATIVE  EXERCISES 

Now  allow  the  water  to  flow  from  the  burette  into  the  pipette 
until  it  has  risen  to  the  right  height  in  the  stem.  Mark  the  tube 
under  the  bulb  on  a  level  with  the  meniscus.  This  will-  com- 
plete the  graduation  of  the  bulb.  A  little  preliminary  measuring 
may  be  necessary  before  commencing  such  a  graduation  in  order 
to  determine  at  what  point  it  is  best  to  locate  the  mark  above 
the  bulb. 

For  the  graduation  of  the  tube  substitute  for  the  pipette 
used  in  graduating  the  bulb  a  5-  or  10-cc.  calibrating  pipette, 
and  find,  as  before,  how  many  stem  divisions  must  be  filled  for 
the  delivery  by  the  instrument  of  5-cc.  (or  10-cc.)  volumes  at 
20°.  Draw  off  exactly  the  required  quantities  and  mark  the 
tube  after  each  withdrawal.  Cover  the  tube  with  wax  and  — 
with  the  aid  of  one  of  the  systems  of  lines  previously  described 
—  graduate  the  spaces  between  the  marks  into  tenths  of  a  cubic 
centimeter.  Finally,  determine  by  a  calibration  of  the  instru- 
ment the  errors  of  the  graduation. 

The  marks  made  upon  the  tube  during  the  graduation  must 
lie,  as  nearly  as  possible,  in  the  same  vertical  line.  They  should 
be  very  short  also,  and  at  the  same  time  so  distinct  that  after- 
wards they  may  be  readily  identified  under  the  wax.  -The  tube 
may  be  waxed  before  graduation,  but  in  that  case  the  covering 
must  be  thin  or  the  tube  will  not  be  sufficiently  translucent. 


In  using  apparatus  graduated  to  deliver  certain  volumes  of 
liquids,  it  is  assumed  that  a  given  area  of  glass  surface  always 
retains  the  same  quantity  of  liquid  when  the  apparatus  is 
emptied,  whereas  the  quantity  retained  may  vary  somewhat 
according  to  the  character  of  the  liquid  and  the  condition  of 
the  surface.  Instruments  of  this  kind  should  be  thoroughly 
cleansed,  first  with  an  alcoholic  solution  of  potassium  hydroxide 
and  then  with  distilled  water.  Grease  of  any  kind  upon  the 
glass  causes  that  portion  of  the  liquid  which  should  remain  uni- 
formly distributed  over  the  surface  to  collect  in  drops,  and  it  is 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS          99 

by  no  means  certain  that  the  quantity  of  liquid  in  these  will  be 
equal  to  that  which  would  normally  cling  to  the  clean  glass. 
Hence  the  interior  of  burettes  and  of  other  liquid-measuring 
apparatus  should  not  be  wiped  with  cloths,  paper,  or  other 
material  likely  to  contain  any  oily  matter.  The  practice  of 
rinsing  with  ether  is  likewise  unsatisfactory  because  the  ordi- 
nary commercial  ether  often  contains  in  solution  fats  or  gums 
which,  after  evaporation  of  the  ether,  are  left  upon  the  glass. 
Sometimes,  in  spite  of  the  most  thorough  cleansing,  the  liquid 
film  persists  in  breaking  and  collecting  in  drops.  In  such  cases 
the  difficulty  cannot  be  remedied  since  it  is  apparently  due  to 
the  character  of  the  glass  surface  and  not  to  any  deposit  of 
foreign  material  upon  it. 

Care  should  be  taken,  when  measuring  from  a  burette,  not  to 
allow  it  to  deliver  too  rapidly,  otherwise  more  than  the  normal 
quantity  of  liquid  will  be  left  temporarily  upon  the  glass,  and 
the  reading,  if  taken  immediately,  will  be  erroneous.  If  there 
is  any  reason  to  fear  an  error  from  this  cause,  the  correctness  of 
the  reading  should  be  confirmed  by  a  later  one. 

The  correct  reading  of  burettes  is  greatly  facilitated  by  the 
use  of  the  Erdmann  float.  The  float  should  be  neither  so  large 
as  to  stick  in  any  part  of  the  tube,  nor  so  small  that  it  can 
assume  any  other  than  a  vertical  position  in  the  liquid. 

Standard  and  Normal  Solutions 

A  standard  solution  is  one  whose  concentration  has  been 
regulated  with  a  view  to  its  convenient  use  in  determining 
other  substances.  For  example,  a  solution  of  potassium  per- 
manganate of  such  concentration  that  one  cubic  centimeter  of 
it  will  convert  five  milligrams  of  iron  from  the  ferrous  to  the 
ferric  condition  is  a  standard  solution ;  likewise  a  solution  of 
sodium  chloride  for  the  determination  of  silver,  one  cubic  centi- 
meter of  which  will  precipitate  ten  milligrams  of  silver.  Any 
solution  used  in  volumetric  work  whose  concentration  is  known 


100  QUANTITATIVE  EXERCISES 

or  whose  equivalence,  with  respect  to  the  substances  which  may 
be  determined  by  it,  has  been  ascertained,  is  a  standard  solution. 
Normal  solutions  are  standard  solutions  which  are  peculiar  in 
this  respect,  that  the  standard  determining  their  concentration 
is  of  such  a  character  that  equal  volumes  of  different  solutions 
are  chemically  equivalent.  The  definitions  for  normal  solutions 
which  are  now  generally  accepted  are  the  following : 

1.  A  normal  solution  of  an  acid  is  one  which  contains  in  a 
liter-volume  one  gram  of  replaceable  hydrogen. 

2.  A  normal  solution  of  a  base  is  one  which  contains  in  a 
liter-volume  that  quantity  of  the  basic  metal  which  will  replace 
one  gram  of  acid  hydrogen. 

3.  A  normal  solution  of  a  salt  is  one  which  contains  in  a  liter- 
volume  that  quantity  of  salt  which  is  formed  by  the  replace- 
ment of  one  gram  of  acid  hydrogen  by  the  metal  of  a  base. 

A  liter- volume  of  a  normal  mono-basic  acid,  or  of  a  normal 
mono-acid  base,  contains  the  number  of  grams  of  the  dissolved 
substance  which  is  equal  to  the  molecular  weight  of  the  sub- 
stance ;  while  a  normal  solution  of  a  bi-basic  acid,  or  of  a  bi-acid 
base,  contains  in  a  liter  the  number  of  grams  of  the  dissolved 
substance  which  is  equal  to  one-half  the  molecular  weight  of 
the  substance;  etc. 

Standard  solutions  of  certain  salts  —  for  example,  potassium 
permanganate  and  ferrous  sulphate  —  are  employed  in  volu- 
metric work  for  purposes  of  oxidation  and  reduction  ;  as  when 
iron  is  determined  in  accordance  with  the  reaction 

2  KMn04  +  10  FeS04  +  8  H2SO4 

=  5  Fe2  3  S04  +  5  H2O  +  K2SO4  +  2  MnSO4 ; 

and  chromic  acid  in  accordance  with  the  reaction 
6  FeSO4  +  2  CrO3  +  6  H2SO4 

=  Cr2  3  SO4  +  3  Fe2  3  SO4  +  6  H2O. 

The  question  arises,  What  are  to  be  considered  as  normal  solu- 
tions in  such  cases?  According  to  the  definitions  previously 
given,  a  normal  solution  of  potassium  permanganate — regarding 


APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS        101 

the  substance  as  a  salt  of  the  mono-basic  acid  HMnO4  —  would 
contain  in  a  liter-volume  the  number  of  grams  of  the  substance 
which  is  equal  to  its  molecular  weight.  But  a  liter  of  such  a 
solution,  acting  as  an  oxidizing  agent,  would  be  equivalent  to 
5  grams  of  hydrogen.  Again,  since  ferrous  sulphate  is  a  salt 
.of  a  bi-basic  acid,  a  normal  solution  of  it,  according  to  the  defi- 
nition, would  contain  in  a  liter-volume  the  number  of  grams  of 
the  substance  which  is  equal  to  one-half  its  molecular  weight. 
But  as  a  reducing  agent,  a  liter  of  such  a  solution  would  be 
equivalent  only  to  one  half  gram  of  hydrogen.  On  the  whole, 
it  appears  more  rational  to  emphasize  the  purpose  for  which 
such  solutions  are  used  rather  than  their  classification  as  salts ; 
and  to  adopt,  therefore,  the  following  definitions  with  reference 
to  them: 

1.  A  normal  solution  of  any  substance  employed  as  an  oxidiz- 
ing agent  is  one  containing  in  a  liter-volume  the  quantity  of 
active  oxygen  which  is  required  to  convert  one  gram  of  hydrogen 
into  water. 

2.  A  normal  solution  of  any  substance  employed  as  a  redu- 
cing agent  is  one  having  in  a  liter-volume  a  reducing  power  equal 
to  that  of  one  gram  of  hydrogen.     According  to  the  definitions 
here  given,  a  normal  solution  of  potassium  permanganate  would 
contain  in  a  liter-volume  the  number  of  grams  of  the  salt  which 
is  equal  to  one-fifth  its  molecular  weight ;  while  a  normal  solu- 
tion of  ferrous  sulphate  would  contain  in  the  same  volume  the 
number  of  grams  of  the  salt  which  is  equal  to  its  molecular 
weight. 

The  principal  advantage  in  using  normal  solutions  in  volu- 
metric work  lies  in  the  fact  that  a  general  system  based  on  the 
atomic  and  molecular  weights  of  the  substances  employed  greatly 
simplifies  the  computation  of  the  quantitative  results  of  all 
chemical  reactions  in  which  those  substances  take  part.  In 
using  standard  solutions  for  the  determination  of  other  sub- 
stances, the  effect  of  unavoidable  errors  of  measurement  upon 
the  correctness  of  the  results  is  proportional  to  the  concentration 


102  QUANTITATIVE  EXERCISES 

of  the  solutions.  It  is  therefore  customary  in  work  requiring 
any  considerable  degree  of  accuracy  to  employ  very  dilute  solu- 
tions. Normal  solutions  are  diluted  to  one-tenth,  arid  in  some 
instances  to  one-hundredth,  of  their  concentration.  The  decimal 
plan  of  dilution  is  adopted  because  it  does  not  sensibly  diminish 
the  advantage  of  the  normal  system. 

Normal  solutions  are  to  be  carefully  distinguished  from  the 
gram-equivalent  solutions  which  are  much  used  in  the  determi- 
nation of  molecular  weights  by  certain  modern  methods.  A 
gram-equivalent  solution  always  contains  in  a  liter  the  number 
of  grams  of  the  dissolved  substance  which  is  equal  to  the  molec- 
ular weight  of  the  substance.  Such  solutions  are  equi-molec- 
ular  or  molecular-equivalent,  that  is,  equal  volumes  of  them 
contain  the  same  number  of  molecules ;  but  the  equal  volumes 
are  not,  as  in  the  case  of  normal  solutions,  necessarily  chem- 
ically equivalent.  Some  confusion  has  arisen  from  the  practice 
of  calling  gram-equivalent  solutions  normal  solutions.  Perhaps 
the  distinction  between  the  two  classes  of  solutions  could  be 
emphasized  with  advantage  by  calling  the  one  volume-normal, 
and  the  other  molecular-normal. 

The  Correction  of  Standard  Solutions  for  Temperature 

All  volumetric  apparatus  should  be  graduated  or  calibrated 
for  the  particular  temperature  which  usually  prevails  in  the 
laboratory  in  which  the  instruments  are  to  be  used.  The  stand- 
ard solutions  also  should  be  so  made  up  as  to  be  correct  at  the 
standard  temperature ;  and  they  should  be  used,  as  far  as  prac- 
ticable, at  that  temperature.  But,  since  the  temperature  of  every 
laboratory  is  subject  to  considerable  variations,  it  is  often  neces- 
sary both  to  make  and  to  use  the  standard  solutions  at  other 
than  the  standard  temperature.  The  errors  which  arise  from 
this  source  are  usually  tolerable  in  ordinary  volumetric  work, 
but  they  require  attention  whenever  any  considerable  degree 
of  accuracy  is  sought.  Such  corrections,  though  seeming  to 


AI'I'AU.VITS    FOll   MKASI'IIKMKNT   OK    UQl  IDS        103 

involve  laborious  computations,  can  be  easily  and  expeditiously 
made  with  the  aid  of  a  very  simple  device  which  will  now  be 
explained. 

In  estimating  the  magnitude  of  the  errors  which  result  from 
measuring  standard  solutions  at  other  than  standard  tempera- 
tures, two  things  must  be  taken  into  account,  —  the  expansion  or 
contraction  of  the  liquids  and  the  expansion  or  contraction  of 
the  measuring  instruments.  The  errors  of  the  measurements 
are  equal  to  the  differences  between  the  two.  We  will  suppose, 
for  purposes  of  explanation,  that  the  standard  temperature  is 
20°,  and  that  water  is  the  liquid  to  be  dealt  with.  The  reason 
for  selecting  water  instead  of  a  solution  will  appear  hereafter. 
The  problem  is  to  devise  some  expeditious  method  of  finding 
what  volumes  quantities  of  water  measured  at  other  tempera- 
tures would  have  if  measured  at  the  standard  temperature  of 
20° ;  or,  if  need  be,  for  finding  what  volumes  quantities  of 
water  measured  at  20°  would  have  if  measured  at  other  tem- 
peratures. We  will  suppose  that  the  extreme  limits  to  be  pro- 
vided for  are  10°  on  the  one  side  and  30°  on  the  other.  The 
following  table  gives  the  volumes  of  water  at  10°,  15°,  25°,  and 
30°  as  compared  with  its  volume  at  20°. 

TABLE  I 

10°  15°  20°  26°  30° 

0.998511  0.999098  1.000000  1.001142  1.002503 

Table  II  gives  the  volumes  which  a  liter  of  water,  measured 
at  20°,  would  have  at  10°,  15°,  25°,  and  30°,  and  the  amounts 
of  the  contraction  or  expansion. 


TABLE  II 

10° 

ir>° 

20° 

25° 

.10° 

8.511  cc. 

MM.  098  cc. 

1000.000  cc. 

1001.142  cc. 

1  002.503  cc. 

1.489  cc. 

0.!M)2  cc. 

0.000  cc. 

1.142  cc. 

2.503  cc. 

Suppose  a  graduated  glass  vessel  holding  exactly  one  liter  at 
20°  is  filled  with  water  also  having  a  temperature  of  20°.     The 


104  QUANTITATIVE  EXERCISES 

volume  of  the  water  will  be  1000  cc.  If  now  the  temperature 
of  the  water  falls  to  10°,  its  volume  will  be  998.511  cc.  (see 
Table  II).  But  the  water  will  not  measure  that  amount  by  the 
graduation  of  the  vessel  because  the  latter  has  also  diminished 

in  size  by  1000  —  ^  AAAOC?  that  is,  by  0.25  cc.    It  will  appear  to 

998.511 

have  a  volume  of  3  -       0~,  or  998.761  cc.;  and  1000  -998.761, 
_L  —  . 


or  1.239  cc.,  is  the  value  of  the  correction  which  must  be  added 
to  the  apparent  volume  to  find  what  the  volume  of  the  water 
would  be  at  20°,  —  the  standard  temperature. 

If,  on  the  other  hand,  the  temperature*  of  the  water  rises  to 
30°,  its  true  volume  will  be  1002.503  cc.  ;  but  it  will  measure 
less  than  that  in  a  graduated  vessel  which  is  correct  at  20° 
because  the  vessel  has  also  suffered  expansion.  Its  apparent 


volume  will  be          '       ,  or  1002.252  cc.;  and  1002.252-1000, 
Ji.OOO^iO 

or  2.252  cc.,  is  the  correction  which  must  be  deducted  from  the 
apparent  volume  of  the  water  at  30°  in  order  to  find  what  its 
volume  at  20°  would  be.  Table  III  gives  the  values  of  the  cor- 
rections for  one  liter  of  water  at  10°,  15°,  25°,  and  30°.  We 
should  obtain  values  sufficiently  near  the  truth  for  all  ordinary 
purposes  by  simply  subtracting  the  contraction  or  expansion  of 
a  liter  measuring  flask  from  the  absolute  contraction  or  expan- 
sion of  a  liter  of  water  as  given  in  Table  II. 

TABLE  III 

10°  15°  25°  30° 

+  1.239  cc.  +  0.777  cc.  -  1.017  cc.  -  2.253  cc. 

For  convenience  of  use  in  practice  a  curve  of  corrections  is 
to  be  made  for  each  degree  between  10°  and  20°  on  one  side 
and  between  20°  and  30°  on  the  other.  To  prepare  the  curves 
a  horizontal  line  upon  the  cross-ruled  paper  is  made  to  represent 
cubic  centimeters  of  liquid  from  0  cc.  to  100  cc.  A  vertical  line 
at  the  100  cc.-end  of  the  horizontal  one,  and  extending  above 


THE 

UNIVERSITY 

RN\^ 

OF  LIQUIDS        105 

and  below  it,  is  employed  for  the  corrections  to  be  applied  to 
a  100  cc. -volume  at  different  temperatures ;  the  upper  portion 
being  reserved  for  positive  quantities  and  the  lower  one  for 
negative  quantities.  Each  space  on  the  vertical  line  is  made 
to  represent  0.01  cc.  Hence  we  cut  off  on  the  upper  portion 
of  it  12.4  spaces  to  represent  the  contraction  from  20°  to  10°, 
and  on  the  lower  portion  22.5  spaces  to  represent  the  expansion 
from  20°  to  30°.  Each  of  these  two  intervals  is  then  divided 
into  ten  equal  parts  labeled  19°,  18°,  17°,  .  .  .  ,  10°  from  the 
base  line  upwards,  and  21°,  22°,  23°,  .  .  .  ,  30°  from  the  base 
line  downwards.  Finally,  the  temperature  points  upon  the 
vertical  line  are  joined  by  straight  lines  to  the  zero  end  of 
the  horizontal  line.  With  the  aid  of  such  a  figure  one  can 
determine  by  a  simple  inspection  how  much  is  to  be  added  to 
or  subtracted  from  any  volume  of  water  not  exceeding  100  cc., 
when  measured  at  any  temperature  between  10°  and  30°,  in 
order  to  find  what  volume  it  would  occupy  at  20° ;  or  he  may 
ascertain  with  equal  ease  what  volume  water  measured  at  20° 
would  appear  to  have  if  measured  at  any  other  temperature 
within  the  given  limits. 

Correction  curves  prepared  in  accordance  with  the  plan  here 
developed  are  not  rigidly  accurate.  The  expansion  of  water 
has  been  assumed  to  be  uniform  between  10°  and  20°  on  the 
one  hand,  and  between  20°  and  30°  on  the  other,  which  is  not 
in  accordance  with  the  truth.  Again,  the  expansion  of  glass 
has  been  assumed  to  be  regular  between  10°  and  30°,  which  is 
also  not  strictly  correct.  The  errors  resulting  from  the  former 
assumption  are  not  large  and  they  may  be  easily  avoided  by 
dividing  the  vertical  or  temperature  line  of  the  correction  figure 
in  accordance  with  the  known  irregularities  of  the  expansion 
of  water.  The  effect  of  the  irregularities  of  the  expansion  of 
glass  is  of  no  significance  since  it  never  amounts  to  quantities 
which  can  be  measured  by  ordinary  volumetric  apparatus. 

The  figures  employed  for  the  correction  of  liquid  volumes 
for  temperature  may,  of  course,  be  drawn  upon  a  larger  scale 


106 


QUANTITATIVE  EXERCISES 


than  that  proposed  above,  and  for  larger  volumes  of  the  liquid, 
e.g.  a  liter  instead  of  100  cc. 

The  expansion  of  glass  and  of  water,  the  usual  solvent,  has 
been  investigated  with  care,  but  the  expansion  coefficients  of 
solutions  of  the  concentration  usually  employed  in  volumetric 
analysis  have  not,  unfortunately,  been  determined  to  any  great 
extent.  It  is  often  assumed  for  purposes  of  correction  that 
these  dilute  solutions  expand  and  contract  with  changing  tem- 
peratures to  about  the  same  extent  as  water  itself.  But,  judging 
by  the  results  of  an  investigation  by  Alfred  Schulze  (ZeitscTirift 
fur  analytisclie  Chemie,  21,  167),  the  assumption  is  justified  only 
in  the  case  of  very  dilute  solutions,  —  solutions,  for  example,  of 
-j1^  normal  concentration.  Schulze  determined  the  expansion 
of  T^  normal  solutions  of  sodium  chloride  and  silver  nitrate 
from  0°  to  30°.  His  results,  together  with  the  corresponding 
figures  for  water,  are  given  for  certain  intervals  of  temperature 

in  the  following  table. 

TABLE  IV 

0°       10°       15°       20°       25°      30° 

Water     1.000000  1.000124  1.000712  1.001615  1.002759  1.004123 
XT 

1.000000  1.000360  1.001030  1.001965  1.003160  1.004630 


8  1.000000  1.000398  1.001075  1.002032  1.003240  1.004690 


In  the  next  table  the  volumes  of  the  three  liquids  at  10°,  15°, 
25°,  and  30°  are  compared  with  their  volume  at  20°. 


Water 


N 


TABLE  V 


io;° 

15° 

20° 

25° 

30° 

0.998511 

0.999098 

1.000000 

1.001142 

1.0X)2503 

0.998398 

0.999066 

1.000000 

1.001192 

1.002659 

0.998369 

0.999044 

1.000000 

1.001205 

1.002652 

If  we  subtract  the  numbers  in  the  above  table  from  unity  and 
multiply  the  differences  by  1000,  and  then  deduct  the  expansion 


APPARATUS   FOR  MEASUREMENT  OF  LIQUIDS        107 

of  a  liter  flask  from  10°  and  15°  to  20°,  and  from  20°  to  25°  and 
30°,  we  shall  obtain  the  quantities  which  would  be  used  in 
constructing  for  a  liter-volume  of  each  of  the  three  liquids  a 
correction  figure  like  that  already  explained.  Table  VI  gives 
these  quantities. 

TABLE  VI 

10°  15°  25°  30° 

Water  +  1.239  cc.         +  0.777  cc.         -  1.017  cc.         -  2.253  cc. 

^  NaCl  1.352  0.809  1.067  2.409 

^  AgNO3  1.381  0.831  1.080  2.402 

Judging  by  the  evidence  afforded  by  this  table,  it  is  probably 
safe  to  employ  the  known  expansion  of  water  for  the  correction 
of  solutions  of  tenth-normal  concentration ;  for  with  a  standard 
temperature  of  20°,  the  maximum  error  which  could  result 
from  so  doing  would  amount  in  measuring  a  liter-volume  of 

^  silver  nitrate  to  only  0.142  cc.  at  10°,  0.054  cc.  at  15°, 
0.063  cc.  at  25°,  and  0.149  cc.  at  30°  ;  while  the  error  in  measuring 
an  equal  volume  of  ^  sodium  chloride  solution  would  amount 

to  only  0.113  cc.  at  10°,  0.032  cc.  at  15°,  0.05  cc.  at  25°,  and 
0.156  cc.  at  30°.  The  maximum  error  in  the  correction  for  a 
measurement  of  50  cc.  of  either  liquid  would  amount  to  less 
than  0.01  cc. 

Schulze  also  determined  the  expansion  of  six  normal  solutions 
from  0°  to  30°.  The  results  for  several  intervals  of  tempera- 
ture, together  with  the  corresponding  volumes  of  water,  are 
given  on  the  following  page. 


108 


QUANTITATIVE  EXERCISES 


Water 
Oxalic  acid 
Hydrochloric 

acid 
Nitric  acid 
Sulphuric  acid 
Sodium  car-    "1 

bonate         j 
Sodium  ") 

hydroxide    j 


TABLE  VII 

0°  10°  15°  20°  25°  30° 

1.000000  1.000124  1.000712  1.001615  1.002759  1.004123 

1.000000  1.000993  1.001940  1.003125  1.004560  1.006168 

1.000000  1.001010  1.001905  1.003013  1.004328  1.005850 

1.000000  1.001800  1.003050  1.004490  1.006120  1.007933 

1.000000  1.001720  1.002945  1.004385  1.005990  1.007820 

1.000000  1.001880  1.003130  1.004565  1.006165  1.007955 

1.000000  1.002050  1.003365  1.004848  1.006510  1.008330 


The  expansion  of  the  six  normal  solutions  included  in  the  table 
diverges  more  from  that  of  water  at  temperatures  below  10° 
than  above.  This  will  appear  more  clearly  if  we  select  20° 
instead  of  0°  as  our  standard  temperature,  as  is  done  in  the 
following  table. 


Water 
Oxalic  acid 
Hydrochloric  acid 
Nitric  acid 
Sulphuric  acid 
Sodium  carbonate 
Sodium  hydroxide 

If  the  quantities  in  Table  VIII  are  treated  as  were  those  in 
Table  V  in  order  to  secure  the  data  required  for  the  construction 
of  a  correction  figure  for  each  of  the  liquids,  we  obtain  the 
following  results. 


Water 
Oxalic  acid 
Hydrochloric  acid 
Nitric  acid 
Sulphuric  acid 
Sodium  carbonate 
Sodium  hydroxide 


10° 
0.998511 
0.997874 
0.998003 
0.997322 
0.997346 
0.997327 
0.997215 

TABLE  VIII 
15° 
0.999098 
0.998818 
0.998895 
0.998566 
0.998566 
0.998571 
0.998524 

20° 
1.000000 
1.000000 
1.000000 
1.000000 
1.000000 
1.000000 
1.000000 

25° 
1.001142 
1.001430 
1.001311 
1.001622 
1.001598 
1.001592 
1.001653 

30° 
1.002503 
1.003033 
1.002828 
1.003427 
1.003420 
1.003374 
1.003465 

TABLE  IX 

10° 

15° 

25° 

30° 

+  1.239  cc. 

+  0.777  cc. 

-  1.017  cc. 

-  2.253  cc. 

1.876 

1.057 

1.305 

2.783 

1.747 

0.980 

1.186 

2.578 

2.428 

1.309 

1.497 

3.177 

2.404 

1.309 

1.473 

3.170 

2.423 

1.304 

1.467 

3.124 

2.535 

1.351 

1.528 

3.215 

APPARATUS  FOR  MEASUREMENT  OF  LIQUIDS        109 

It  appears  from  the  values  recorded  in  Table  IX  that  the 
known  expansion  of  water  cannot  be  employed  for  the  correction 
of  standard  solutions  of  normal  concentration.  Up  to  the  pres- 
ent time  we  have  the  necessary  data  for  the  temperature  correc- 
tion of  only  the  six  normal  solutions  included  in  the  above  table. 
As  a  temporary  expedient,  however,  it  is  probably  safe  to  employ 
for  normal  solutions  of  unknown  expansion  the  mean  values  of 
the  corrections  in  Table  IX.  For  a  liter-volume  these  are : 

TABLE  X 

10°  15°  20°  25°  30° 

+  2.236  cc.         +  1.218  cc.         0.000  cc.    -  1.409  cc.         -  3.008  cc. 

We  have  to  consider  now  the  course  to  be  followed  when  a 
standard  solution  is  to  be  made  at  some  other  than  the  standard 
temperature.  We  will  suppose  that  the  standard  temperature 
is  20°  and  that  a  tenth-normal  solution  of  sodium  chloride  is  to 
be  made  at  15°  in  a  liter  flask,  which  is  correct,  of  course,  only  at 
20°.  At  15°  the  flask  will  hold  not  a  liter,  but  1.000  -=-  1.000125, 
or  999.875  cc.,  and  this  will  be  the  true  volume  of  the  solution 
when  made  up  at  the  given  temperature.  We  wish  now  to  find 
what  its  volume  at  20°  will  be.  For  this  purpose  we  assume 
that  the  expansion  of  tenth-normal  solutions  is  about  equal  to 
that  of  water.  If  the  volume  of  water  at  20°  is  1.0,  its  volume 
at  15°  is  0.999098  (see  Table  I);  therefore  999.875  cc.  of  the 
solution  measured  at  15°  will  become  999.875  ---  0.999098,  or 
1000.778  cc.,  at  20°.  A  liter  of  the  solution  should  contain  at  the 
standard  temperature  5.806  grams  of  the  salt,  and  a  cubic  centi- 
meter 0.005806  gram.  The  weight  of  the  salt  to  be  dissolved 
at  15°  is  therefore  0.005806  x  1000.778,  or  5.8105  grams. 

Suppose,  on  the  other  hand,  that  the  temperature  at  which 
the  solution  is  to  be  made  is  25°.  The  flask  will  then  have  a 
capacity  of  1.000  x  1.000125,  or  1000.125  cc.,  and  this  will  be  the 
volume  of  the  solution  when  made.  Its  volume  at  20°  will  be 
1000.125  -=-1.001142,  or  998.98  cc.  The  quantity  of  the  salt  to  be 
weighed  out  is  therefore  998.98  x  0.005806,  or  5.8001  grams. 


CHAPTER   V 

THE  PREPARATION  OF  STANDARD  SOLUTIONS  OF  ACIDS 
AND  ALKALIES 

INDICATOES 

When  a  standard  solution  of  potassium  permanganate  is 
allowed  to  flow  from  a  burette  into  a  solution  of  ferrous  sul- 
phate to  which  a  little  sulphuric  acid  has  been  added,  the  fol- 
lowing reaction  takes  place : 

2  KMn04  +  10  FeS04  +  8  H2SO4  =  K2SO4-f  2  MnSO4 

+  5  Fe23S04  +  8  H2O. 

As  long  as  there  remains  any  ferrous  salt  in  the  solution,  the 
permanganate  is  reduced  and  loses,  in  consequence,  its  charac- 
teristic color.  As -soon,  however,  as  the  reaction  with  the  irpn 
is  finished,  the  reduction  ceases,  and  an  additional  drop  of  the 
permanganate  —  or  even  a  fraction  of  a  drop  if  the  volume  of 
the  liquid  is  not  large — imparts  a  distinct  rose  color  to  the  solu- 
tion. The  sudden  appearance  of  the  color  is  so  striking  that 
one  is  always  able  to  determine  with  great  precision  the  point 
at  which  the  reaction  between  the  permanganate  and  the  ferrous 
iron  is  finished. 

If  a  standard  solution  of  silver  nitrate  is  added  in  the  same 
way  to  a  solution  of  any  chloride,  the  chlorine  is  precipitated  as 
silver  chloride, 

AgN03  +  NaCl  =  NaNO3 + AgCl. 

But  since  the  solution  of  silver  nitrate  is  colorless,  and  the 
freshly  formed  silver  chloride  is  slow  in  subsiding,  it  is  diificult 
to  determine  when  the  precipitation  is  completed.  If,  however, 
in  the  beginning,  there  are  added  to  the  solution  of  chloride  a 

110 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     111 

few  drops  of  a  solution  of  neutral  potassium  chromate,  the  pre- 
cipitation of  silver  chloride  will  proceed  as  before,  and  there 
will  be  no  permanent  reaction  between  the  silver  nitrate  and 
the  chromate  until  the  precipitation  of  the  chlorine  is  finished. 
There  will  then  appear  in  the  solution  a  permanent  red  color  due 
to  the  formation  of  silver  chromate,  which  is  a  sufficient  indica- 
tion that  enough  of  the  standard  solution  has  been  added. 

Again,  if  a  solution  of  sodium  arsenite  is  added  to  one  of 
bleaching  powder,  the  arsenite  will  be  converted  into  arseniate, 

Na3  A  sO3  +  CaCl2O  =  Na3  AsO4  +  CaCl2. 

But  nothing  will  occur  in  the  solution  to  indicate  when  the 
reaction  is  finished.  If,  however,  a  drop  of  the  liquid  is  removed 
upon  the  end  of  a  glass  rod  from  time  to  time,  and  applied  to 
a  piece  of  filter  paper  which  has  been  dipped  in  a  solution  of 
starch  and  potassium  iodide  and  then  tlried,  it  is  easy  to  ascer- 
tain when  the  last  trace  of  hypochlorite  disappears  ;  for  as  long 
as  any  of  it  is  present  a  blue  spot  will  appear  whenever  the 
paper  is  touched  with  a  drop  of  the  liquid. 

Substances  which,  like  potassium  chromate  in  the  determina- 
tion of  chlorine  by  a  standard  solution  of  silver  nitrate,  and  like 
the  mixture  of  potassium  iodide  and  starch  in  the  determination 
of  a  hypochlorite  by  sodium  arsenite,  are  employed  to  indicate 
the  completion  of  reactions  are  called  indicators.  Only  those 
which  are  most  frequently  used  in  connection  with  the  neutral- 
ization of  acids  and  bases  need  be  considered  in  this  place. 

1.  LITMUS 

Preparation.  The  crushed  commercial  litmus  is  repeatedly 
extracted  with  fresh  quantities  of  boiling  85  per  cent  alcohol 
for  the  purpose  of  removing  a  violet  coloring  matter  which  is 
reddened  by  acids  but  not  made  blue  by  alkalies.  The  residue, 
consisting  mainly  of  calcium  carbonate,  carbonates  of  the  alka- 
lies, and  the  substance  to  be  isolated,  is  washed  with  more  hot 


112  QUANTITATIVE  EXERCISES 

alcohol  upon  a  filter,  and  then  digested  for  several  hours  with 
cold  distilled  water.  The  filtered  aqueous  extract  has  a  pure 
blue  color  and  contains  an  excess  of  alkali,  a  part  of  which  is 
in  the  form  of  carbonate  and  a  part  in  combination  with  the 
litmus.  To  remove  the  alkaline  reaction,  the  solution  is  heated 
to  the  boiling  point  and  cautiously  treated  with  very  dilute  sul- 
phuric acid  until  it  becomes  distinctly  and  permanently  red. 
The  boiling  must  be  continued  until  the  carbonic  acid  which  is 
liberated  in  the  solution  has  been  expelled,  otherwise  the  blue 
color  may  reappear  after  a  time.  The  red  solution,  after  expul- 
sion of  the  carbonic  acid,  is  treated  with  a  dilute  solution  of 
barium  hydroxide  until  the  color  changes  to  a  violet,  and  is 
then  filtered.  In  this  state  the  solution  is  very  sensitive  to 
acids  or  alkalies,  changing  to  red  with  the  former  and  to  blue 
with  the  latter. 

In  closed  vessels  the 'solution  soon  loses  color  and  acquires 
an  offensive  odor.  But  if  a  solution  which  has  been  thus 
altered  is  exposed  to  the  air  in  shallow  vessels,  it  soon  regains 
its  original  color.  Solutions  of  litmus  should  therefore  be  kept 
in  open  and  only  partly  filled  bottles.  Paper  caps  placed  over 
the  necks  of  the  bottles  will  protect  the  solutions  from  dust 
and  at  the  same  time  admit  the  air  which  is  necessary  for  their 
preservation. 

In  neutralizing  an  acid  with  an  alkali,  the  quantity  of  the  lat- 
ter which  must  be  added  is  somewhat  more  than  equivalent  to 
the  quantity  of  the  former,  since  a  small  amount  of  the  alkali 
is  required  to  give  the  litmus  a  blue  color.  For  the  same 
reason,  when  an  alkali  is  neutralized,  a  slight  excess  of  the  acid 
must  be  added.  Hence  the  importance  of  making  the  indicator 
as  sensitive  as  possible  in  the  beginning.  There  are  two  reme- 
dies for  the  difficulty.  The  litmus  solution  may  be  divided  into 
two  parts,  one  of  which  is  changed  to  a  red  color  with  the  least 
possible  quantity  of  acid,  and  the  other  to  a  blue  with  the  least 
possible  quantity  of  alkali.  When  now  an  acid  is  to  be  neutral- 
ized with  a  standard  alkali,  the  blue  solution  is  employed  as  the 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     113 

indicator  and  vice  versa.  Or,  one  may  apply  a  so-called  color 
correction.  The  magnitude  of  this  correction  is  ascertained  by 
adding  to  a  quantity  of  neutral  water,  equal  in  volume  to  the 
solution  of  acid  or  alkali,  the  same  amount  of  the  indicator  that 
is  to  be  used  in  the  neutralization  experiment,  and  then  finding 
how  much  of  the  standard  solution  is  required  to  produce  the 
required  color. 

If  preferred,  the  litmus  may  be  precipitated  and  preserved  in 
solid  form.  For  this  purpose  the  solution,  prepared  as  described 
above,  is  evaporated  to  a  small  volume  and  then  treated  with 
strong  alcohol.  The  precipitate  is  collected  on  a  filter,  washed 
with  alcohol,  and  dried  over  calcium  chloride. 

Litmus  is  a  satisfactory  indicator  with  the  following  sub- 
stances : 

1.  Hydrochloric,  hydrobromic,  sulphuric,  nitric,  and  oxalic 
acids. 

2.  The  hydroxides  of  the  alkaline  and  alkaline  earth  metals 
and  ammonia. 

3.  The  arsenites  and  silicates  of  the  alkalies. 

4.  The  carbonates  and  sulphides  of  the  alkalies  and  alkaline 
earths  in  boiling  solutions. 

Its  conduct  is  unsatisfactory : 

1.  With  sulphurous,  phosphoric,  arsenic,  boric,  and  chromic 
acids. 

2.  With  most  organic  acids. 

3.  With  carbonates  and  sulphides  in  cold  solutions. 

2.  PHENOLPHTHALEIN 

Preparation.  Half  a  gram  of  the  solid  material  is  dissolved 
in  100  cc.  of  neutral  95  per  cent  alcohol.  If  the  alcohol  is 
found  to  have  an  acid  reaction,  as  it  often  has,  it  should  be 
boiled  for  a  short  time.  If  its  acid  reaction  does  not  disappear 
after  boiling,  the  alcohol  should  be  agitated  with  dry  calcium 
hydroxide  (slaked  lime)  and  redistilled. 


114  QUANTITATIVE  EXERCISES 

Phenolphthalein  is  nearly  colorless  in  neutral  and  acid  solu- 
tions and  red  in  those  containing  the  least  excess  of  alkali.  It  is 
a  delicate  indicator: 

1.  For  the  hydroxides  of  the  alkaline   and  alkaline  earth 
metals. 

2.  For  most  mineral  acids. 

3.  For  most  organic  acids. 
Its  conduct  is  unsatisfactory : 

1.  With  ammonia  and  in  solutions   containing  ammonium 
salts. 

2.  With  arsenious,  silicic,  and  boric  acids. 
Phenolphthalein  is  exceedingly   sensitive    to    carbonic  and 

sulphhydric  acids,  but  exhibits  a  neutral  reaction  as  soon  as 
their  acid  salts  have  been  formed.  With  phosphoric  and  arsenic 
acids  it  exhibits  a  neutral  reaction  when  two-thirds  of  their  acid 
hydrogen  has  been  replaced.  In  this  respect  it  differs  from 
litmus,  toward  which  the  mono-hydrogen  phosphates  and  arseni- 
ates  of  the  alkaline  metals  are  alkaline,  and  the  di-hydrogen 
salts  neutral. 

3.  METHYL  ORANGE 

Preparation.  One  part  of  the  dyestuff  is  dissolved  in  a 
thousand  parts  of  cold  distilled  water.  Ordinarily  not  more 
than  two  or  three  drops  of  this  dilute  preparation  should  be 
added  to  a  solution  in  which  a  determination  is  to  be  made. 

In  very  dilute  neutral  or  alkaline  solutions  methyl  orange 
has  a  yellow  color  which  changes  to  a  pink  in  the  presence  of 
the  least  excess  of  one  of  the  stronger  mineral  acids.  It  owes 
its  great  value  as  an  indicator  not  to  any  superior  sensitive- 
ness, but  principally  to  the  fact  that  it  is  almost  wholly  indif- 
ferent to  carbonic  and  sulphhydric  acids,  and  to  certain  salts 
which  give  an  acid  reaction  with  litmus  and  some  other  indi- 
cators. It  is  also  indifferent  to  arsenious,  boric,  silicic,  and 
hydrocyanic  acids,  which  makes  it  practicable  to  determine  vol- 
umetrically  the  bases  in  combination  with  these  weak  acids. 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     115 

Methyl  orange  may  be  used  as  an  indicator  in  the  determina- 
tm  of  the  following  substances : 

1.  The  stronger  mineral  acids. 

2.  The  hydroxides  of  the  alkaline  and  alkaline  earth  metals. 

3.  Ammonia. 

4.  The  bases  in  such  carbonates,  sulphides,  arsenites,  silicates, 
ad  borates  as  are  decomposed  by  dilute  nitric,  hydrochloric,  or 
alphuric  acid. 

5.  Phosphoric  and  arsenic   acids  which  become  neutral  to 
lethyl  orange    when   one-third   of   their   hydrogen   has  been 
^placed. 

6.  Sulphurous  acid  which  becomes  neutral  to  the  indicator 
*ith  the  formation  of  its  acid  salt. 

Methyl  orange  cannot  be  used  for  the  determination  of  organic 
cids,  or  in  solutions  containing  nitrous  acid  or  nitrites  which 
estroy  it.  Moreover,  it  cannot  be  employed  in  hot  solutions. 

4.  TKOP^OLIN 

The  dyestuff  known  as  tropseolin  No.  00  conducts  itself 
owards  acids  and  alkalies  like  methyl  orange. 

5.  COCHINEAL 

Preparation.  Commercial  cochineal  is  reduced  to  a  coarse 
powder  and  digested  with  from  10  to  20  parts  of  25  per  cent 
alcohol. 

In  neutral  and  acid  solutions  cochineal  has  a  yellowish-red 
color,  which  changes  to  a  violet  in  the  presence  of  alkali. 
Towards  the  stronger  mineral  acids  and  the  hydroxides  of  the 
alkaline  and  alkaline  earth  metals  it  is  quite  sensitive.  It  is 
of  little  value  in  the  titration  of  organic  acids,  and  cannot  be 
used  in  solutions  containing  acetates,  salts  of  copper,  or  any 
traces  of  iron  or  aluminium.  It  can  be  used  by  gaslight  with 
very  satisfactory  results. 


116  QUANTITATIVE  EXERCISES 


EXERCISE  X 

THE   PREPARATION   OF    STANDARD    SOLUTIONS    OF    ACIDS 
AND   ALKALIES 

I.   BY  MEANS  OF  OXALIC  ACID 
a.  Preparation  of  Pure  Oxalic  Acid 

Agitate  commercial  oxalic  acid  in  a  closed  bottle  with  a  mix- 
ture of  equal  parts  of  ether  and  95  per  cent  alcohol  until  the 
liquid  is  fully  saturated  with  the  acid.  Filter  into  a  flask 
through  a  plaited  paper.  Partly  immerse  the  flask  in  a  water 
bath,  connect  it  with  a  condenser  and  receiver,  and  then  heat  as 
long  as  a  copious  distillate  is  obtained.  To  the  distillate,  which 
consists  mainly  of  ether,  add  more  alcohol,  and  with  the  mixture 
extract  more  of  the  acid.  Filter  and  distill  as  before.  When 
enough  of  the  acid  has  been  extracted,  and  its  solution  freed 
from  ether,  add  to  the  residue  an  equal  volume  of  distilled 
water  which  is  free  from  ammonia  and  concentrate  upon  the 
water  bath  in  a  porcelain  dish  until  the  odor  of  ethyl  oxalate 
and  alcohol  disappears  and  the  solution  is  ready  to  crystallize 
on  cooling.  If  the  odor  of  ethyl  oxalate  or  alcohol  has  not 
wholly  disappeared  when  the  solution  becomes  sufficiently  con- 
centrated for  crystallization,  more  water  must  be  added  and  the 
evaporation  repeated.  Stir  the  solution  while  the  crystals  are 
forming.  Collect  the  acid  in  a  funnel  in  the  bottom  of  which 
a  perforated  porcelain  disk  or  a  platinum  cone  has  been  placed. 
Remove  as  much  as  possible  of  the  mother  liquor  with  the  aid 
of  a  filter  pump,  and  spread  the  acid  to  dry  upon  a  clean  unglazed 
porcelain  plate.  Burn  a  quantity  of  the  acid  upon  the  lid  of  a 
platinum  crucible,  or  in  a  dish  of  the  same  material,  to  find 
whether  or  not  it  leaves  a  weighable  residue. 

Commercial  oxalic  acid  contains  acid  oxalates  from  which  it 
cannot  be  readily  freed  by  the  ordinary  process  of  recrystalliza- 
tion  from  water.  The  difficulty  in  removing  the  salts  by  this 


STANDARD  SOLUTIONS  OF  ACIDS  AKD  ALKALIES     117 

method  is  due  to  the  fact  that  the  acid  oxalates,  though  readily 
dissolved  by  hot  water,  are  only  very  moderately  soluble  in  cold 
water.  Hence  they  are  reprecipitated  from  cooling  saturated 
solutions  of  the  acid.  A  partial  removal  of  the  salts  may  be 
effected  by  saturating  cold  water  with  the  acid  and  then  evap- 
orating to  the  point  of  crystallization. 

The  air-dried  acid  is  neither  deliquescent  nor  efflorescent  in 
air  containing  the  usual  amount  of  moisture.  It  cannot  be  dried 
in  a  desiccator,  since  in  an  atmosphere  devoid  of  moisture  it  loses 
water  of  crystallization.  Its  solutions  are  not  altogether  stable. 

b.  Preparation  of  a  Tenth-Normal  Solution  of  Oxalic  Acid 

Oxalic  acid  is  bi-basic  and  has  a  molecular  weight  of  125.08. 
It  would  therefore  be  necessary,  in  order  to  prepare  a  liter  of 

125  08 
the  normal  solution,  to  weigh  out  — ^ — ,  or  62.54  grams  of  the 

A 

pure  acid.  To  prepare  a  liter  of  a  tenth-normal  solution,  one- 
tenth  of  this  quantity,  or  6.254  grams,  would  be  required.  One 
half-liter  of  the  latter  solution  will  suffice  for  the  present 
exercise.  This  will  require  3.127  grams  of  the  acid.  The 
weighing  of  fixed  quantities,  however,  consumes  much  time. 
It  is  therefore  better  to  proceed  as  follows :  Weigh  out  a  few 
milligrams  more  than  the  required  amount  of  the  acid.  Dis- 
solve it  in  distilled  water,  free  from  ammonia  and  carbonic  acid ; 
transfer  the  solution  to  a  half-liter  flask,  taking  care  not  to  wet 
the  neck  ;  rinse  the  vessel  in  which  the  solution  was  made  sev- 
eral times  with  water  and  pour  the  rinsings  into  the  flask. 
Add  more  water,  a  little  at  a  time,  mixing  the  contents  of  the 
flask  after  each  addition,  without  wetting  the  neck,  until  the 
flask  is  nearly  full.  Now  fill  to  the  mark,  close  the  flask,  and 
mix  the  liquid  thoroughly.  The  solution  will  be  somewhat  too 
concentrated.  Divide  the  weight  of  acid  required  for  a  half- 
liter,  i.e.  3.127  grams,  by  the  weight  of  the  acid  in  one  cubic 
centimeter  of  the  solution.  This  will  give  the  number  of  cubic 


118  QUANTITATIVE  EXERCISES 

centimeters  which  must  be  diluted  to  half  a  liter,  and  the  differ- 
ence between  that  number  and  500  will  be  the  volume  of  water 
required  for  the  dilution.  With  a  pipette  remove  a  quantity 
of  the  solution,  add  the  required  volume  of  water,  and  then  fill 
the  flask  to  the  mark  with  a  portion  of  the  solution  which  was 
removed  to  make  room  for  the  water. 

The  plan  here  proposed  for  making  solutions  of  definite  con- 
centration from  weighed  quantities  of  substances  has  various 
advantages  over  that  of  weighing  out  exactly  the  required 
amounts  of  the  substances  to  be  dissolved.  The  most  important 
of  these  are  the  saving  of  time  and  the  avoidance  of  long  expo- 
sure of  the  material  to  the  atmosphere  at  the  time  of  weighing. 
Such  exposure  at  the  balance  of  powdered  materials,  or  of  those 
which  absorb  water  or  carbonic  acid,  is  a  serious  obstacle  in  the 
way  of  obtaining  correct  weighings. 

There  is  one  error  in  the  process  which  requires  attention 
when  the  solutions  to  be  made  are  of  considerable  concentra- 
tion. It  is  assumed  that  the  final  volume  of  the  liquid  is  equal 
to  the  sum  of  the  volumes  of  the  solution  diluted  and  of  the 
water  which  is  added  to  it,  whereas,  in  the  case  of  aqueous 
solutions,  dilution  is  usually  attended  by  contraction.  The 
effect  of  the  error  upon  tenth-normal  solutions  is  unimportant, 
and  in  the  case  of  more  concentrated  solutions  it  may  be  reduced 
to  insignificance  by  weighing  out  only  very  little  more  than  the 
required  amount  of  the  substances  to  be  dissolved. 

Distilled  water  usually  contains  ammonia  and  carbonic  acid, 
both  of  which  must  be  removed  before  it  is  used  in  making 
standard  solutions  of  acids.  Therefore  the  water  which  is  to 
be  employed  for  this  purpose  should  be  vigorously  boiled  for  20 
or  30  minutes  and  then  allowed  to  cool  to  the  standard  temper- 
ature. Carbonic  acid  in  the  water  with  which  standard  solu- 
tions of  acids  are  made  increases  the  acidity  and  interferes  with 
the  normal  conduct  of  such  indicators  as  litmus  and  phenol- 
phthalein ;  while  ammonia,  under  the  same  conditions,  weakens 
the  acids  and  strengthens  the  standard  alkalies.  If  only  those 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     119 

indicators  are  used  which  are  indifferent  to  carbonic  acid,  the 
presence  of  that  acid  is,  of  course,  unobjectionable.  But  stand- 
ard acids  which  are  made  up  with  water  containing  consider- 
able carbonic  acid  will  exhibit  different  concentrations  when 
neutralized  with  an  alkali,  according  as  litmus  or  phenolphthal- 
ein  on  the  one  hand,  or  methyl  orange  on  the  other,  is  used  as 
the  indicator.  It  is  therefore  advisable  to  expel  the  carbonic 
acid  and  ammonia  from  all  water  which  is  to  be  used  in  connec- 
tion with  neutralization  experiments.  The  boiling  of  the  water 
for  this  purpose  should  take  place  in  porcelain  vessels,  since 
hot  water  extracts  alkali  from  all  varieties  of  glass  which  are 
used  in  a  chemical  laboratory,  and  from  some  of  them  with 
astonishing  rapidity.  This  source  of  error  in  acidimetric  work 
should  not  be  overlooked.  Beakers,  flasks,  and  other  glass 
vessels  should  be  'tested  with  regard  to  their  fitness  for  use  in 
such  experiments.  This  may  be  done  by  boiling  in  them  for 
an  hour  or  more  water  having,  in  the  beginning,  a  neutral  reac- 
tion ;  and  then  determining  —  with  the  aid  of  phenolphthalein 
and  a  very  dilute  standard  acid  —  whether  an  appreciable 
amount  of  alkali  has  been  dissolved. 

The  common  practice  of  employing  compressed  air  from  the 
lungs  to  force  from  a  wash  bottle  the  water  which  is  required 
for  dilution  or  other  purposes  is,  of  course,  inadmissible  in  .vol- 
umetric work  with  acids  and  bases. 


c.  Preparation  of  a  Tenth-Normal  Alcoholic  Solution  of  Potassium 

Hydroxide 

Dissolve  two  or  three  grams  of  sodium  hydroxide  in  a  liter 
and  a  half  of  95  per  cent  alcohol  and  redistill.  This  treatment 
will  free  the  alcohol  from  carbonic  and  other  acids  which 
the  commercial  material  usually  contains,  and  also  from  'alde- 
hydes and  a  substance  extracted  from  the  oak  barrel  staves, 
both  of  which  when  treated  with  alkali  give  a  yellow  color  to 
the  alcohol. 


120  QUANTITATIVE  EXERCISES 

Dissolve  about  3.3  grams  of  the  purest  obtainable  potas- 
sium hydroxide  in  a  half-liter  of  the  redistilled  alcohol,  and 
set  the  solution  aside  in  a  glass-stoppered  bottle.  At  first  the 
solution  will  be  cloudy  from  the  presence  in  it  of  insoluble 
potassium  carbonate,  but  this  soon  subsides  and  becomes  firmly 
attached  to  the  glass,  leaving  the  solution  of  hydroxide  quite 
clear. 

With  the  room  and  the  solutions  at  very  nearly  the  standard 
temperature,  fill  one  burette  with  the  tenth-normal  oxalic  acid, 
and  another  with  the  alcoholic  caustic  potash.  Cover  both 
burettes  with  inverted  test  tubes.  Measure  into  an  Erlenmeyer 
flask  25  cc.  of  the  standard  acid  and  add  to  it  a  few  drops  of  a 
solution  of  phenolphthalein  which  has  also  been  made  with 
the  redistilled  alcohol.  Titrate  the  acid  with  the  alkali  until 
the  red  color  appears  and  then  determine  whether  a  measurable 
quantity  of  the  acid  is  required  to  destroy  it.  Titrate  back  and 
forth  with  the  two  solutions  until  a  point  is  reached  where  the 
quantities  of  alkali  and  acid  required  to  produce  and  destroy 
the  color  cannot  be  measured  upon  the  burettes.  Repeat  the 
experiment.  If  the  first  and  second  results  agree,  calculate 
from  the  ascertained  relation  of  the  two  solutions  how  many 
cubic  centimeters  of  the  alkali  will  be  required  to  make  a  half- 
liter  of  a  tenth-normal  solution.  Measure  into  a  500-cc.  flask 
the  volume  of  alcohol  necessary  for  dilution,  and  fill  to  the 
mark  with  the  alkali.  Compare  the  diluted  solution  with  the 
standard  oxalic  acid.  They  should  be  found  equivalent,  vol- 
ume for  volume. 

The  principal  objection  to  the  use  of  phenolphthalein  as  an 
indicator  is  its  extreme  sensitiveness  to  carbonic  acid.  Solu- 
tions containing  it  which  have  been  reddened  by  the  addition 
of  a  slight  excess  of  an  alkaline  hydroxide  soon  lose  their 
color  in  the  air  in  consequence  of  the  absorption  of  carbon 
dioxide.  Even  the  breath  of  the  operator  is  often  a  source 
of  difficulty  in  a  titration  when  this  indicator  is  used.  The 
evil  is  partially  remedied  by  using  flasks  instead  of  beakers, 


STANDARD  SOLUTIONS  OF   ACIDS  AND  ALKALIES     121 

and  by  titrating,  whenever  practicable,  into  hot  instead  of  cold 
solutions. 

Alcoholic  solutions  of  the  alkaline  hydroxides  are  not  stable 
and  must  therefore  be  frequently  restandardized.  They  soon 
turn  yellow  and  then  reddish-brown  in  consequence,  apparently, 
of  the  absorption  of  oxygen  and  the  formation  of  aldehyde  resin. 
Any  moderate  development  of  color  does  not,  however,  interfere 
with  their  use  in  neutralizing  acids.  The  principal  advantage 
in  using  an  alcoholic  rather  than  an  aqueous  solution  of  an 
alkaline  hydroxide  lies  in  the  fact  that  the  alkaline  carbonates, 
being  nearly  insoluble  in  concentrated  alcohol,  separate  from 
the  former  as  fast  as  they  are  formed  in  consequence  of  unavoid- 
able exposure  of  the  solutions  to  the  air.  In  other  words,  alco- 
holic solutions  of  the  caustic  alkalies  keep  themselves  free  from 
carbonates  and  therefore  in  fit  condition  for  use  with  indicators 
which,  like  litmus  and  phenolphthalein,  are  sensitive  to  car- 
bonic acid. 

Aqueous  solutions  of  the  alkaline  hydroxides  which  are  prac- 
tically free  from  carbonates  may  be  prepared  by  saturating  so- 
called  absolute  alcohol  with. the  solid  caustic  alkalies  and  then 
diluting  the  alcoholic  solutions,  after  subsidence  of  the  insol- 
uble matter,  with  water.  In  such  solutions  any  carbonate 
which  may  be  subsequently  formed  in  consequence  of  exposure 
to  the  air  will,  of  course,  remain  in  solution. 

Aqueous  solutions  of  the  caustic  alkalies  may  be  freed  from 
carbonates  by  adding  to  them  a  little  hydroxide  of  calcium  or 
of  barium.  But  such  solutions,  as  will  be  explained  hereafter, 
cannot,  under  certain  conditions,  be  used  in  conjunction  with 
oxalic  acid. 

The  great  change  in  volume  which  alcohol  suffers  when  its 
temperature  is  raised  or  lowered  constitutes  an  objection  to  the 
employment  of  alcoholic  solutions  in  volumetric  work.  Care 
must  be  taken  both  to  make  and  to  use  such  solutions  at  the 
standard  temperature.  Otherwise  corrections  must  be  applied 
to  the  measurements. 


122  QUANTITATIVE   EXERCISES 

If  the  volume  of  alcohol  (96  to  97  per  cent)  at  0°  is  repre- 
sented by  unity,  its  volume  at  any  higher  temperature  t  will 
be  found  by  the  formula 

1  +  at  +  bt 2  +  ct 8,  in  which 
a  =  0.00104139 
b  =  0.0000007836, 
c  =  0.000000017618. 

The  following  table  gives  the  volumes  of  alcohol,  also  those 
of  water  at  several  temperatures. 


TABLE  I 

Alcohol 
Water 

0° 
1.000000 
1.000000 

10° 
1.010510 
1.000124 

15° 
1.015857 
1.000712 

20° 
1.021282 
1.001615 

25° 
1.026799 
1.002759 

30° 
1.032423 
1.004123 

The  next  table  gives  the  volumes  of  alcohol  and  of  water  at 
10°,  15°,  25°,  and  30°  as  compared  with  their  volumes  at  20°. 

TABLE  II 

10°  15°  20°  25°  30° 

Alcohol        0.989452         0.994688         1.000000         1.005403         1.010908 
Water          0.998511         0.999098         1.000000         1.001142         1.002503 

Table  III  gives,  in  cubic  centimeters,  the  decrease  in  volume 
of  a  liter  of  alcohol  and  of  a  liter  of  water  when  their  tempera- 
ture falls  from  20°  to  15°  and  10°  ;  and  their  increase  in  vol- 
ume when  the  temperature  rises  from  20°  to  25°  and  30°. 

TABLE  III 

10°  15°  20°  25°  30° 

Alcohol  10.55  cc.  6.31  cc.          0.00  cc.          5.40  cc.  10.91  cc. 

Water  1.49  cc.          0.90  cc.          0.00  cc.  1.14  cc.  2.50  cc. 

If  we  deduct  from  the  volumes  given  in  the  first  horizontal 
line  of  the  table  the  contraction  of  a  liter  flask  from  20°  to  15° 
and  10°  and  its  expansion  from  20°  to  25°  and  30°,  we  shall 
obtain  the  data  required  for  the  preparation  of  a  correction 
figure  (see  page  104)  to  be  used  in  connection  with  strong  alcohol 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     123 

or  with  weak   solutions   of  alkalies   in   concentrated  alcohol. 
Table  IV  gives  these  differences. 

TABLE  IV 

10°  15°  20°  25°  30° 

10.3  cc.  6.185  cc.  0.00  cc.  5.275  cc.  10.66  cc. 

d.  Preparation  of  a  Tenth-Normal  Solution  of  Hydrochloric  Acid 

Dilute  25  cc.  of  concentrated  chemically  pure  hydrochloric 
acid  to  a  liter  volume  with  boiled  distilled  water.  Determine 
the  acid  in  measured  portions  of  the  solution  with  the  standard 
alcoholic  caustic  potash,  using  phenolphthalein  as  the  indicator, 
and  then,  without  lurther  dilution,  proceed  to  determine  the 
strength  of  the  acid  as  directed  under  II. 

II.  BY  MEANS  OF  CARBONATES 

Dissolve  about  6.5  grams  of  potassium  hydroxide  in  a  liter 
of  distilled  water.  The  solution  will  be  somewhat  more  than 
decinormal.  It  is  not  necessary  to  know  its  exact  strength,  nor 
is  it  necessary  to  boil  the  water  with  which  it  is  made. 

Weigh  into  beakers  or  Erlenmeyer  flasks  two  portions  of 
pure  calcium  carbonate  of  about  0.2  gram  each.  Add  to  each 
portion  50  cc.  of  the  hydrochloric  acid  whose  strength  was 
determined  as  directed  under  I,  d,  and  cover  the  beakers  or 
flasks  with  watch  glasses.  When  the  solution  of  the  carbonate 
—  which  may  be  hastened  by  the  application  of  a  very  gentle 
heat  —  is  complete,  rinse  the  covering  glasses  into  the  solutions 
with  distilled  water,  add  a  few  drops  of  a  dilute  solution  of 
methyl  orange,  and  titrate  the  excess  of  hydrochloric  acid  with 
the  water  solution  of  potassium  hydroxide.  Having  found 
how  much  of  the  latter  is  required  to  neutralize  the  former, 
measure  out  an  equal  volume  of  the  alkali  and  titrate  it  with 
the  hydrochloric  acid  whose  strength  is  to  be  determined.  The 
difference  between  the  volume  of  the  acid  required  to  neutralize 


124  QUANTITATIVE  EXERCISES 

the  alkali  and  that  added  to  the  carbonate  in  the  first  place 
is,  of  course,  the  volume  of  the  acid  which  has  been  neutralized 
in  dissolving  the  carbonate.  From  this  and  from  the  known 
weight  of  the  carbonate  it  is  easy  to  calculate  the  concentration 
of  the  hydrochloric  acid.  The  two  results  should  agree  very 
closely  with  each  other  and  with  those  obtained  by  proceeding 
as  directed  under  I,  d. 

Dilute  the  acid  to  the  decinormal  standard  (using  boiled 
distilled  water  for  the  purpose)  and  compare  the  diluted  solu- 
tion with  the  decinormal  oxalic  acid,  using  for  this  purpose  the 
standard  alcoholic  solution  of  potassium  hydroxide  and  phenol- 
phthalein.  Equal  volumes  of  the  two  acids  should  neutralize 
the  same  volume  of  the  alkali. 

Saturate  a  small  quantity  of  so-called  absolute  alcohol  with 
potassium  hydroxide,  and  when  the  liquid  has  become  clear 
dilute  10  cc.  of  the  solution  to  a  liter  with  recently  boiled 
water.  With  the  aid  of  this  solution  compare  again  the  two 
decinormal  acids,  using  at  one  time  phenolphthalein  and  at 
another  litmus  as  the  indicator. 

Calcium  carbonate  for  the  standardization  of  acids  may  be 
prepared  in  the  following  manner:  Calcium  chloride,  such  as 
is  usually  employed  for  drying  gases,  is  dissolved  in  water; 
the  solution  is  treated  with  a  little  calcium  hydroxide,  boiled, 
and  filtered;  the  calcium  is  then  precipitated  as  oxalate  with 
ammonium  oxalate  and  a  little  ammonia.  After  standing  in  a 
warm  place  for  several  hours  the  oxalate  is  collected  on  a  filter, 
washed  with  water,  dried,  and  converted  into  carbonate  by  heat- 
ing it  in  a  platinum  dish.  The  carbonate  is  dissolved  in  hydro- 
chloric acid  and  the  calcium  reprecipitated  as  oxalate  ;  and  from 
the  oxalate  the  carbonate  is  obtained  in  the  same  manner  as 
before.  The  carbonate  thus  obtained  will  probably  have  a  gray 
color  in  consequence  of  the  separation  of  carbon.  It  is  dissolved 
in  hydrochloric  acid  and  the  solution,  if  not  perfectly  clear,  is 
filtered.  The  calcium  is  now  precipitated  as  carbonate  with 
ammonia  and  ammonium  carbonate.  The  precipitate  is  collected, 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     125 

thoroughly  washed  with  boiling  water,  dried,  transferred  to  a 
platinum  dish,  moistened  with  a  solution  of  ammonium  carbon- 
ate, and  heated  to  constant  weight  at  a  temperature  below  a  red 
heat.  The  treatment  with  ammonium  carbonate  and  the  subse- 
quent heating  to  constant  weight  should  be  repeated  until  such 
repetition  is  found  to  produce  no  change  in  the  weight  of  the 
material. 

The  crystallized  calcium  carbonate  known  as  Iceland  spar  is 
often  pure  enough  to  be  used  in  determining  the  strength  of 
acids,  but  much  of  the  material  now  sold  under  that  name  con- 
tains magnesium  carbonate.  It  is  therefore  unsafe  to  employ 
a  specimen  of  the  mineral  for  this  purpose  until  its  purity  has 
been  demonstrated. 

OTHER  METHODS  OF  STANDARDIZING  ACIDS  AND  ALKALIES 

In  general,  any  substance  of  pronounced  acid  or  basic  charac- 
ter which  can  be  obtained  in  pure  condition  and  is  stable  in  the 
air  may  be  employed  to  standardize  alkalies  and  acids. 

Of  the  acids  which  satisfy  these  conditions,  none  are  superior 
to  oxalic  acid.  There  are,  however,  several  acid  salts  which 
may  be  used  with  advantage  for  the  purpose,  such  as  the  acid 
oxalates  of  the  alkali  metals  and  acid  potassium  tartrate.  The 
compound  of  this  class  which  is  most  frequently  employed  is 
potassium  tetroxalate. 

Of  the  basic  substances  suitable  for  the  determination  of  the 
strength  of  acids,  the  carbonates  of  sodium  and  of  the  alkaline 
earths  are  probably  the  best,  though  certain  oxides,  e.g.  the 
oxide  of  zinc,  may  be  used. 

The  concentration  of  acids  may  be  determined  by  precipi- 
tating and  weighing  their  insoluble  salts,  such  as  barium  sul- 
phate, silver  chloride,  and  silver  bromide.  There  are  also  certain 
reactions  which  are  attended  by  the  liberation  or  the  consumption 
of  definite  quantities  of  acid,  and  some  of  these  may  be  utilized 
to  produce  an  acid  of  known  strength,  or  to  ascertain  the 


126  QUANTITATIVE  EXERCISES 

strength  of  an  acid  of  unknown  concentration.  An  example 
of  such  reactions  is  furnished  by  the  following  equation : 

Na2S03  +  H20  +  I2  =  Na2S04  +  2  HI. 

If  the  quantity  of  the  iodine  which  takes  part  in  the  reaction  is 
known,  the  quantity  of  the  resulting  hydriodic  acid  can  be  cal- 
culated, and  this  may  be  employed  to  determine  the  neutraliz- 
ing power  of  any  solution  of  an  alkali. 

A  few  of  the  methods  of  standardizing  alkalies  and  acids  will 
be  given  in  greater  detail. 


1.  By  Means  of  Potassium  Tetroxalate 

Potassium  tetroxalate,  KHC2O4  •  H2C2O4-  2  H2O,  may  be  used 
with  advantage  instead  of  oxalic  acid.  The  salt  is  stable  in  the 
air  at  ordinary  temperatures  ;  but,  like  oxalic  acid,  it  cannot  be 
dried  in  a  hot-air  bath  or  in  a  desiccator.  It  is  best  prepared 
in  the  following  manner:  A  concentrated  solution  of  oxalic 
acid  is  divided  into  two  parts,  one  of  which  is  slightly  less  than 
a  fourth  of  the  whole.  The  smaller  portion  is  heated  to  the 
boiling  point,  treated  with  a  little  litmus,  and  carefully  neutral- 
ized with  pure  potassium  carbonate.  The  larger  portion  is  then 
heated  and  the  solution  of  potassium  oxalate  stirred  into  it.  If 
any  tetroxalate  separates  during  the  mixing  of  the  liquids,  the 
heating  is  continued,  with  addition  of  more  water  if  neces- 
sary, until  a  clear  solution  is  obtained.  The  solution  is  stirred 
while  crystallizing  to  prevent  the  formation  of  large  crystals, 
which  are  much  more  apt  than  the  smaller  ones  to  include 
mother  liquor.  The  product  should  be  recrystallized  once  or 
twice  from  pure  distilled  water.  The  crystals  are  collected  in  a 
funnel,  in  the  b6tto,m  of  which  a  platinum  cone  or  a  perforated 
porcelain  disk  has  been  placed,  and  freed  to  the  greatest  possi- 
ble extent,  with  the  aid  of  a  filter  pump,  from  mother  liquor. 
Finally,  they  are  washed  with  a  little  cold  water  and  dried  in 
the  air  upon  unglazed  porcelain  plates. 


STANDARD  SOLUTIONS  OF  ACIDS  AND  ALKALIES     127 

The  tetroxalate  is  a  tri-basic  acid  with  a  molecular  weight  of 
252.22 ;  a  normal  solution  of  it  would,  therefore,  contain  in  a 
liter- volume  84.0733  grams  of  the  salt. 

2.  By  Means  of  Sodium  Carbonate 

The  substance  which  has  been  longest  and  probably  most 
frequently  employed  for  the  standardization  of  acids  is  the 
neutral  anhydrous  carbonate  of  sodium.  It  is  prepared  for  use 
either  from  the  acid  salt  or  from  the  crystallized  neutral  car- 
bonate by  heating  in  a  platinum  vessel  until  everything  volatile 
has  been  expelled.  The  product  is  hygroscopic  and  must  there- 
fore be  preserved  in  closed  bottles.  The  portion  of  the  material 
which  is  to  be  used  in  any  experiment  should  be  reheated  in  a 
small  platinum  crucible  or  boat,  cooled  in  a  desiccator,  and  pro- 
tected from  the  moisture  of  the  air  while  weighing  by  inclosing 
it  in  a  weighing  glass.  If  an  indicator  is  used  which,  like  lit- 
mus and  phenolphthalein,  is  sensitive  to  carbonic  acid,  the  neu- 
tralization of  the  carbonate  must  take  place  in  a  boiling  solution. 

3.  By  Means  of  Sulphuric  Acid  derived  from  Copper  Sulphate 

If  an  aqueous  solution  of  copper  sulphate  is  subjected  to 
electrolysis,  it  is  quantitatively  decomposed  into  metallic  copper 
and  free  sulphuric  acid.  It  is  evident  that  if  the  sulphate  is 
pure  and  the  weight  of  it  is  known,  the  quantity  of  the  acid 
which  will  be  liberated  can  be  calculated ;  also  that  this  known 
quantity  of  acid  may  be  utilized  to  determine  the  strength  of 
any  alkaline  solution,  which  may  in  turn  be  employed  to 
standardize  other  acids.  The  sulphate  should  be  purified  by 
repeated  crystallizations.  It  is  stable  in  the  air  at  ordinary 
temperatures,  but  cannot  be  dried  in  a  desiccator  or  in  a  hot-air 
bath.  The  electrolysis  may  be  effected  in  a  platinum  dish 
with  precipitation  of  the  copper  upon  the  platinum,  or  in  a 
beaker  glass  with  use  of  any  of  the  ordinary  forms  of  platinum 
electrodes. 


CHAPTER  VI 

THE   DETERMINATION  OF   SPECIFIC  GRAVITY 
DENSITY  AND  SPECIFIC   GRAVITY 

The  terms  density  and  specific  gravity,  as  they  are  ordinarily 
employed,  have  the  same  significance,  though  the  former  is 
usually  applied  to  gases  and  the  latter  to  liquids  and  solids. 
Both  are  used  to  signify  the  amount  of  matter  in  an  object,  or 
its  mass,  as  compared  with  the  mass  of  an  equal  volume  of  some 
standard  substance.  If  any  distinction  between  the  terms  is  to 
be  attempted,  we  might  define  density  as  the  mass  of  a  unit 
volume  of  any  object,  and  specific  gravity  as  the  weight  of  a 

unit  volume  of  the  same;  in  which  case  density  =  — -, ,  and 

volume 

specific  gravity  =  —  p^ —     But  this  distinction  is  immaterial 
volume 

since  in  all  the  practice  of  the  chemist  the  terms  mass  and 
weight  have  fundamentally  the  same  significance.  In  a  certain 
sense  the  term  specific  gravity  is  misleading,  for  it  suggests 
the  erroneous  idea  that  the  density  of  an  object  is  in  some 
way  dependent  on  the  force  of  gravity,  which  is  a  variable 
quantity. 

Water  at  4°  C.  has  been  selected  as  the  standard  for  the 
comparison  of  liquids  and  solids  with  respect  to  specific  gravity 
or  density.  If  the  cubic  centimeter  and  the  gram  are  employed 
as  the  units  of  volume  and  mass,  the  unit  of  specific  gravity 
and  that  of  weight  are  in  all  respects  identical,  since  both  are 
the  cubic  centimeter  of  water  at  4°.  The  specific  gravity  of  any 
liquid  or  solid  may  then  be  defined  as  the  weight  in  a  vacuum 
of  one  cubic  centimeter  of  the  substance,  and  conversely,  the 

128 


THE  DETERMINATION  OF  SPECIFIC  GRAVITY       129 

specific  volume  of  any  liquid  or  solid  may  be  defined  as  the 
volume  of  one  gram  of  the  substance. 

Owing  to  the  lightness  of  gases  as  compared  with  other  forms 
of  matter  and  also  to  the  simple  relation  existing  between  their 
weights  and  those  of  the  molecules  of  which  they  are  made  up, 
it  is  more  convenient  to  employ  some  gas  as  the  standard  of 
density  for  this  class  of  bodies.  Hydrogen  is  usually  made  the 
standard  for  comparison.  Accordingly  the  density  of  a  gas  may 
be  defined  as  its  weight  in  a  vacuum  as  compared  with  the 
weight  of  an  equal  volume  of  hydrogen  when  measured  under 
the  same  conditions  of  temperature  and  pressure.  The  molecu- 
lar weight  of  any  gas  may  then  be  found  by  doubling  its  density, 
since  hydrogen  with  a  density  of  1  has  a  molecular  weight  of  2, 
and  the  molecular  weights  of  all  gases  are  proportional  to  their 
densities. 

Air  is  also  employed  as  a  standard  for  the  comparison  of 
gases  with  respect  to  density.  It  is,  however,  ill  adapted  to  the 
purpose  owing  to  the  variability  of  its  composition. 

The  density  of  an  object  changes  with  its  volume  and  its 
volume  varies  with  its  temperature.  It  is  therefore  necessary 
to  fix  upon  a  standard  temperature  not  only  for  the  standard 
substance  but  also  for  all  of  those  substances  which  are  to  be 
compared  with  it.  As  stated  already,  the  standard  temperature 
for  water  (the  standard  substance)  is  4°, —  the  temperature  of 
its  maximum  density.  The  standard  temperature  for  other  sub- 
stances is  0°.  A  strict  adherence  to  the  latter  standard  is,  how- 
ever, practicable  only  in  the  case  of  those  substances  whose 
cubical  expansion  coefficients  are  known.  Hence  in  many 
instances  it  is  possible  only  to  give  the  specific  gravity  of  a 
liquid  or  solid  for  the  particular  temperature  at  which  it  was 
experimentally  determined. 

The  temperature  at  which  all  gases  are  compared  with  respect 
to  density  is  0°.  Owing  to  their  compressibility  there  is  required 
for  gaseous  bodies  a  second  standard,  —  that  of  pressure.  The 
standard  pressure  is  that  which  will,  at  0°,  support  a  vertical 


130  QUANTITATIVE  EXERCISES 

column  of  mercury  760  mm.  in  length.  But,  since  the  pressure 
required  to  support  such  a  column  of  mercury  varies  with  the 
intensity  of  the  earth's  attraction,  it  is  necessary  to  define  the 
latitude  and  altitude  at  which  it  shall  be  valid  as  a  standard. 
The  latitude  and  altitude  agreed  upon  are  45°  and  sea  level. 


EXERCISE  XI 

DETERMINATION  OF  THE   SPECIFIC  GRAVITY  OF  SOLIDS 
I.    DETERMINATION  OP  THE  SPECIFIC  GRAVITY  OF  A  SILVER  COIN 

There  are  required: 

1.  A  thoroughly  cleansed  silver  dollar  or  half  dollar. 

2.  A  fine  platinum  wire  long  enough  to  suspend  the  coin  at 
the  proper  distance  above  the  balance  pan. 

3.  A  beaker  glass  of  recently  boiled  distilled  water  which  has 
cooled  to  the  temperature  of  the  balance  room. 

4.  A  bridge  to  place  over  the  balance  pan  as  a  support  for 
the  beaker. 

Weigh  the  coin  and  then  suspend  it  in  a  loop  on  the  end  of 
the  wire  at  the  proper  height  above  the  pan.  Remove  the  coin 
from  the  loop,  leaving  the  wire  in  its  place  on  the  hook.  Place 
the  beaker  of  water  on  the  bridge  over  the  pan  and  weigh  the 
wire  with  the  loop  submerged  in  the  water.  Remove  the  wire, 
replace  the  coin  in  the  loop,  suspend  it  in  a  beaker  of  hot  water, 
and  then  boil  until  all  air  is  expelled  from  the  surface  of  the 
coin.  Immerse  the  coin  for  a  few  minutes  in  cold  water,  and 
then  transfer  it,  still  wet,  to  the  water  on  the  bridge.  Weigh 
the  coin  submerged  in  the  water  and  note  the  temperature  of 
the  latter. 

The  difference  between  the  last  weight  and  that  of  the  wire 
with  its  loop  submerged  is  the  weight  of  the  coin  in  water. 

The  difference  between  the  weight  of  the  coin  in  air  and  in 
water  is  the  weight  of  a  volume  of  the  water  equal  to  that  of 
the  coin. 


THE  DETERMINATION    OF  SPECIFIC  GRAVITY        131 

Correct  the  weight  of  the  coin  and  that  of  the  water  displaced 
by  it  to  a  vacuum,  using  0.0012  gram  as  the  weight  of  a  cubic  cen- 
timeter of  air;  8.4  and  21.5  as  the  specific  gravities  of  the  brass 

j     i  *.•  •  i  A  i.-     i  weight  of  coin 

and  platinum  weights  respectively;   — r- j— 

weight  of  displaced  water 

as  a  sufficiently  close  approximation  to  the  specific  gravity  of 
the  coin ;  and  the  proper  value,  to  be  found  in  the  tables,  as  the 
density  of  the  water. 

Divide  the  corrected  weight  of  the  coin,  by  the  corrected 
weight  of  the  water.  The  quotient  is  the  specific  gravity  of 
the  former  as  compared  with  water  at  the  temperature  of  the 
experiment.  To  find  the  specific  gravity  of  tlie  coin  as  com- 
pared with  water  at  4°,  the  result  must  be  multiplied  by  the 
density  of  the  water  at  the  time  of  weighing  the  coin  in  it.  An 
exact  determination  would  also  involve  a  correction  for  the 
expansion  of  the  coin  from  0°  to  the  temperature  of  the  water 
at  the  time  of  the  experiment;  but  since  its  expansion  coeffi- 
cient is  unknown,  this  correction  cannot  be  applied. 

II.    DETERMINATION  OF  THE  SPECIFIC  GRAVITY  OF  GLASS 

IN  FRAGMENTS 
There  are  required : 

1.  A  specific-gravity  flask  with  a  perforated  stopper  (Fig.  20). 

2.  From  10  to  15  grams  of  broken  glass. 

Cleanse  the  flask  first  with  an  alcoholic  solution  of  caustic 
potash  and  then  with  distilled  water.    Drain  it  thoroughly  and 
wipe  the  outside  with  filter  paper.   Insert  a  cork  through 
which  have  been  passed  a  filled  calcium  chloride  tube 
and  a  small  glass  tube  which  reaches  nearly  to  the 
bottom  of  the  flask.     Aspirate  air  through  the  flask 
until  it  is  dry.    Weigh  the  flask  by  the  method  of 
Borda  or  Gauss,  —  first  empty  and  then  with  the  glass 
whose  specific  gravity  is  to  be  determined.     Cover 
the  glass  with  distilled  water.     Place  the  flask  under  a  bell 
jar  which  is  connected  with  an  air  pump  and  exhaust  until  the 


132  QUANTITATIVE  EXERCISES 

air  has  been  removed  from  the  surface  of  the  glass.  Fill  the  flask 
with  recently  boiled  distilled  water  and  insert  the  stopper.  Place 
it  in  a  water  bath  having  a  temperature  a  few  degrees  above  that 
of  the  balance  room.  Stir  the  water  in  the  bath  and  keep  it  for  a 
long  time  at  a  constant  temperature,  still  above  that  of  the  balance 
room,  by  adding  water  of  a  higher  temperature  whenever  neces- 
sary. Remove  the  flask  from  the  bath,  carefully  cleanse  the  out- 
side  with  filter  paper,  and  then  allow  it  to  stand  for  an  hour  or 
more  in  the  balance  case.  Weigh  by  the  same  -method  as  before. 
Remove  the  broken  glass,  fill  the  flask  with  distilled  water, 
heat  in  the  bath  to  the  same  temperature  as  before,  remove  the 
flask,  cleanse,  cool  and  weigh  it  as  in  the  first  instance. 
Let  P  represent  the  weight  of  the  flask, 

P,  the  weight  of  the  flask  and  the  glass, 

W  the  weight  of  the  flask  when  filled  with  glass  and 

water,  and 

W,  the  weight  of  the  flask  when  filled  with  water  alone. 
Then  P,-P  is  the  weight  of  the  glass  a,  and  (P,  -P )  + 
W  —  W  is  the  weight  of  the  water  displaced  by  the  glass  b. 

Reduce  a  and  b  to  their  weights  in  a  vacuum,  using  -  as  the 

specific  gravity  of  the  glass,  and  divide  the  corrected  value  of 
a  by  the  corrected  value  of  b.  The  quotient  will  be  the  den- 
sity of  the  glass  as  compared  with  that  of  water  when  both  have 
the  temperature  of  the  bath.  We  wish,  however,  to  compare 
glass  at  0°  with  water  at  4°.  For  the  purpose  of  explaining 
the  method  of  applying  the  temperature  corrections,  we  will 
assume  that  the  temperature  of  the  bath  was  25°  and  let  a  and 
b  represent  the  corrected  weights  of  the  glass  and  of  the  dis- 
placed water.  The  commonly  accepted  cubical-expansion  coef- 
ficient of  glass  is  0.000025.  Accordingly  a  unit  volume  of 
glass  at  0°  becomes  1.000625  volumes  at  25°.  A  unit  volume 
of  water  at  4°  becomes  1.002868  volumes  at  25°.  Now  if  we  sup- 
pose the  volumes  of  the  glass  and  of  the  water  to  be  kept  constant 
by  the  addition  of  more  material  while  the  temperature  of  the 


THE  DETERMINATION  OF  SPECIFIC   GRAVITY        133 

former  falls  from  25°  to  0°,  and  that  of  the  latter  from  25°  to 
4°,  the  final  weight  of  the  glass  will  be  a,  x  1.000625  grams, 
and  that  of  the  water  displaced  will  be  bt  x  1.002868  grams. 
Hence  the  specific  gravity  of  the  glass  at  0°  as  compared  with 

,0     ...  ,     a,     1.000625 
water  at  4    will  be  -  x  ' 


The  student  should  familiarize  himself  with  the  use  of  Jolly's  balance  and 
of  Nicholson's  hydrometer  for  the  determination  of  the  specific  gravity  of  solids. 
He  should  also  work  out  in  detail  : 

1.  A  method  for  determining,  by  weighing  in  water,  the  specific  gravity  of  a 
substance  which  is  lighter  than  water. 

2.  A  method  for  determining,  by  weighing  in  benzene  (sp.  gr.  0.88),  the  spe- 
cific gravity  of  a  substance  which  is  soluble  in  water. 

3.  A  method  of  determining  the  specific  gravity  of  a  substance  by  measuring 
the  water,  or  other  liquid,  which  it  displaces. 


EXERCISE  XII 

DETERMINATION   OF   THE   SPECIFIC   GRAVITY  OF  LIQUIDS 
I.  WITH  THE  PYCNOMETER 

There  are  required: 

1.  An  Ostwald  (Fig.  21)  or  Sprengel  (Fig.  22)  pycnometer. 

2.  A  quantity  of  pure  benzene. 

Wet  the  outside  of  the  instrument  with  distilled  water  and 
then  dry  it  with  filter  paper.  Suspend  it  by  a  platinum  wire 
from  the  hook  over  the  balance 
pan,  and  weigh.  Attach  to  b 
a  rubber  tube,  which  is  in  turn 
connected  with  a  full  pipette, 

—  or  with  any  other  arrange- 
ment to  prevent  the  entrance 
of  saliva  into  the  rubber  tube, 

—  and  immerse  the  end  a  in 
recently  boiled  distilled  water 

which  has  cooled  to  the  temperature  of  the  room.     Fill  the 
apparatus  by  moderately  diminishing  the  pressure  within  and 


Tl 


0=t 


if 


FIG.  21 


FIG.  22 


134  QUANTITATIVE  EXERCISES 

suspend  it  by  the  platinum  wire  in  a  bath  of  distilled  water  hav- 
ing a  temperature  slightly  above  that  of  the  air.  Maintain  the 
bath  for  a  considerable  time  at  a  constant  temperature  as  directed 
in  Exercise  XI  under  II.  Just  before  removing  the  instrument 
from  the  bath,  touch  the  point  a  with  filter  paper  until  the  liquid 
in  the  tube  recedes  to  the  mark  c.  If,  by  accident,  too  much 
liquid  is  withdrawn,  the  difficulty  may  be  remedied  by  touching 
the  end  of  the  tube  a  with  a  drop  of  water  on  a  glass  rod. 
Remove  the  pycnometer  from  the  bath,  dry  it  with  filter  paper, 
and  weigh  when  its  temperature  has  fallen  to  that  of  the  balance 
room.  Empty  the  pycnometer,  rinse  with  successive  small  por- 
tions of  strong  alcohol,  then  with  ether  which  leaves  no  residue 
on  evaporation,  and  dry  by  aspirating  air  through  it. 

Fill  the  apparatus  with  benzene  and  proceed  in  exactly  the 
same  manner  as  when  it  was  filled  with  water,  being  very  care- 
ful to  maintain  the  bath  at  the  same  temperature  as  before. 

Having  found  the  relative  weights  (reduced  to  a  vacuum)  of 
equal  volumes  of  water  and  benzene  at  the  temperature  of  the 
bath,  calculate  the  specific  gravity  of  the  latter  at  0°  as  com- 
pared with  the  former  at  4°,  using  0.00138  as  the  cubical 
expansion  coefficient  of  benzene. 

II.   BY  WEIGHING  AN  OBJECT  IN  Two  LIQUIDS 

Construct  a  glass  sinker,  Fig.  23,  with  a  little  mercury  in 
the  bottom  to  increase  its  weight  and  to  keep  it  in  a  vertical 
position  when  submerged  in  a  liquid.  The  instrument 
should  have  a  volume  of  from  10  to  15  cc.  Place  in  the 
balance  case  two  well-covered  cylinders,  one  containing 
distilled  water  and  the  other  pure  benzene.  Suspend 
IG'  the  sinker  by  a  platinum  wire  from  the  stirrup  hook  and 
weigh  it,  first  in  the  air,  next  in  the  water,  and  finally  in  the 
hydrocarbon,  taking  care  that  the  same  length  of  wire  is  sub- 
merged in  both  liquids.  Note  the  temperature  of  the  water  and 
of  the  benzene,  which,  of  course,  should  be  the  same. 


THE  DETERMINATION  OF  SPECIFIC  GRAVITY        135 

Let  W  represent  the  weight  of  the  sinker  in  air, 
Ww  its  weight  in  water, 
Wb  its  weight  in  benzene. 

Then  W—  Ww  is  the  weight  of  the  water  displaced  by  the 
sinker,  and 

W—  Wbis  the  weight  of  an  equal  volume  of  benzene. 
Reduce  the  weights  to  their  values  in  a  vacuum  and  find  the 
density  of  benzene  at  0°  as  compared  with  that  of  water  at  4°. 


III.   BY  THE  MOHR-WESTPHAL  BALANCE 

The  principle  employed  in  the  preceding  experiment  is  util- 
ized in  a  convenient  manner  in  the  construction  of  the  Mohr- 
Westphal  balance,  Fig.  24.  There  are  several  modifications 
of  the  instrument,  but  in  its  usual  form  it  consists  essentially 
of  a  beam  without  pans.  One 
arm  is  shorter  and  heavier  than 
the  other  and  serves  only  as  a 
counterpoise.  The  longer  and 
lighter  arm  is  graduated  into 
ten  equal  divisions.  At  the 
end  of  the  graduated  arm  is 
suspended  by  a  platinum  wire 
a  thermometer  which  serves  as 
a  sinker.  The  adjustment  is 
such  that  the  balance  is  in  equi- 
librium when  the  sinker  is  sus- 
pended in  air. 

The  weights  are  of  three  or  four  denominations  in  decimal 
order.  The  heaviest  one  is  just  equal  to  the  loss  in  weight 
which  the  sinker  suffers  when  it  is  immersed  in  water  having  a 
temperature  of  15°.  Hence  if  the  sinker  is  submerged  in  water 
of  that  temperature  and  one  of  the  heaviest  weights  is  placed 
upon  the  hook  from  which  the  sinker  is  suspended,  the  balance 
will  again  be  in  equilibrium. 


FIG.  24 


136  QUANTITATIVE  EXERCISES 

To  determine  at  the  same  temperature  the  specific  gravity  of 
a  liquid  heavier  than  water,  one  of  the  heaviest  weights  is 
placed  upon  the  hook,  the  sinker  is  immersed  in  the  liquid,  and 
other  weights  are  then  distributed  along  the  arm  until  equilib- 
rium is  obtained.  Suppose  we  represent  the  weights  in  their 
descending  order  by  Av  A%,  B,  and  (7,  and  that  in  a  given 
experiment  the  distribution  along  the  arm,  when  the  balance  is 
in  equilibrium,  is  as  follows:  Al  on  the  10th  division  (i.e.  on 
the  hook),  Al  on  the  5th,  A2  on  the  3d,  and  B  on  the  9th. 
The  specific  gravity  of  the  liquid  is  1.539. 

When  the  specific  gravity  of  a  liquid  lighter  than  water  is  to 
be  determined,  the  weight  A1  is  removed  from  the  hook.  Care 
must  be  taken  that  the  depth  to  which  the  platinum  wire  is 
submerged  in  the  different  liquids  is  always  the  same. 


HYDROMETERS  (AREOMETERS,  DENSIMETERS) 

Instruments  of  this  kind  are  constructed  in  accordance  with 
the  principle  that  a  body  will  sink  in  a  liquid  which  is  specific- 
ally lighter  than  itself  until  it  has  displaced  its  own  weight  of 
the  liquid. 

There  are  two  classes  of  hydrometers.  In  one  of  these  the 
immersion  of  the  instrument  is  constant,  and  in  the  other  vari- 
able. As  examples  of  the  first  class,  we  have  the  hydrometers 
of  Fahrenheit  and  of  Nicholson  ;  and  of  the  second,  the  volu- 
meter of  Gay-Lussac,  the  ordinary  specific-gravity  spindles,  the 
hydrometers  of  Beaume  and  of  Twaddell,  alcoholometers,  etc. 

Of  these  two  classes  of  hydrometers,  examples  of  which  are 
described  in  the  following  pages  under  A  and  B,  only  instru- 
ments of  the  seconoT  variety,  i.e.  those  of  variable  immersion,  are 
employed  by  the  chemist.  Instruments  belonging  to  the  first 
class,  i.e.  those  of  constant  immersion,  are,  however,  of  interest 
to  him  from  the  standpoint  of  the  principles  which  are  involved 
in  their  construction. 


THE  DETERMINATION  OF   SPECIFIC   GRAVITY        137 

CLASS  A 

B 

Instruments  of  Constant  Immersion 

1.  FAHRENHEIT'S  HYDROMETER 

This  instrument,  Fig.  25,  is  made  wholly  of  glass.  It  con- 
sists (1)  of  a  small  bulb  #,  which  is  filled  with  mercury  to 
keep  the  apparatus  in  a  vertical  position  when  swimming  in  a 
liquid  ;  (2)  of  a  larger  bulb  5,  which  gives  to  the  instru- 
ment the  necessary  volume ;  (3)  a  narrow  tube  <?,  on 
which  is  made  a  mark  showing  to  what  depth  the  instru- 
ment is  to  be  immersed ;  (4)  a  cuplike  enlargement  of 
the  tube  c?,  which  serves  as  a  receptacle  for  the  weights 
by  which  the  requisite  degree  of  submersion  is  effected. 

If  the  hydrometer  is  placed  in  water  of  any  required 
temperature,  e.g.  15°,  and  weights  are  added  to  the  cup      <^# 
until  the  instrument  sinks  in  the  liquid  to  the  mark  on  FIG  25 
the  stem,  it  is  clear  that  the  weight  of  the  water  dis- 
placed will  be  equal  to  the  sum  of  the  weights  of  the  instrument 
and  of  the  load  placed  in  the  cup. 

Let  W  represent  the  weight  of  the  displaced  water, 

P  the  weight  of  the  instrument  (previously  ascertained), 

and 
P,  the  sum  of  the  weights  in  the  cup. 

Then  W=P  +  P,. 

Suppose  the  hydrometer  to  be  placed  in  any  other  liquid  of 
the  same  temperature,  and  to  be  sunk  to  the  mark  by  the  addi- 
tion of  the  required  weights.  The  volume  of  the  liquid  dis- 
placed will  be  the  same  as  before. 

Let  IF,  represent  the  weight  of  the  displaced  liquid, 
P,,  the  sum  of  the  weights  in  the  cup. 

Then  W,  =  P+P^,  and  the  specific  gravity  of  the  liquid  for  the 
given  temperature,  as  compared  with  that  of  water  at  the  same 

W, 
temperature,  is  — '• 


138  QUANTITATIVE  EXERCISES 

«. 

2.  NICHOLSON'S  HYDROMETER 

Nicholson's  hydrometer,  Fig.  26,  differs  from  Fahrenheit's  in 
that  it  is  usually  made  of  metal,  and  has  suspended  from  its 
lower  end  a  basket  or  bucket  in  which  a  solid  substance  may 
be  weighed  under  a  liquid.  It  is  chiefly  used  for  the  deter- 
mination of  the  specific  gravity  of  solids.  Its  weight  is  so 
adjusted  that  a  fixed  additional  weight,  e.g.  100  grams, 
is  required  to  sink  the  instrument  to  the  mark  in  water 
of  standard  temperature,  e.g.  15°.  To  determine  the 
specific  gravity  of  a  solid,  the  substance  is  placed  in 
the  cup  and  weights  are  added  until  the  hydrometer 
sinks  to  the  mark  in  water  of  standard  temperature. 
The  object  is  then  transferred  to  the  basket  below  and 
more  weights  are  added  until  the  hydrometer  sinks  to 
the  same  point  as  before. 

Let  P  represent  the  fixed  weight, 

Pf  weights  added  when  the  substance  was  in  the  cup, 
Pfl  the  sum  of  the  weights  in  the  cup  when  the 

substance  was  in  the  basket, 
W  the  weight  of  the  substance  in  the  air,  and 
F       6  Wf  the  weight  of  the   water  displaced  by  the 

substance. 
Then  W=P-Pf,  Wf=Pf,-Pf,  and  the  specific  gravity  of 

W 

the  object  is  —  • 
Wl 

CLASS  B 

Instruments  of  Variable  Immersion 
1.  GAY-LUSSAC'S  VOLUMETER 

The  hydrometer  of  Gay-Lussac  is  called  a  volumeter  because 
the  principle  employed  in  its  construction  is  such  that  one  reads 
upon  its  scale  the  volume  of  that  portion  of  the  instrument 
which  is  immersed  in  a  liquid.  The  glass  tube  represented  in 


THE   DETERMINATION  OF  SPECIFIC   GRAVITY        139 

Fig.  27  is  supposed  to  be  of  uniform  diameter  throughout,  to 
be  closed  at  the  lower  end,  and  to  be  graduated  upward  from  0 
to  200  in  divisions  of  equal  length.     If  such  a  tube  is  placed  in 
water  of  any  standard  temperature  and  weighted  with 
mercury  until  it  sinks  to  the  100  mark  and  is  then  trans- 
ferred to  a  heavier  liquid  of  the  same  temperature  in 
which  it  sinks  to  the  80  mark,  it  is  clear  that  80  vol- 
umes of  the  heavier  liquid  have  the   same  weight  as 
100  volumes  of  the  water;  also  that  the  specific  gravity 

of  the  former  is  — — -,  or  1.25,  since  the  densities  of  dif- 
80 

ferent  substances  are  inversely  proportional  to  the  vol- 
umes of  equal  weights. 

An  instrument  of  the  form  represented  in  Fig.  27  FIG  27 
would  not  be  sufficiently  sensitive  owing  to  the  great 
width  of  the  tube  bearing  the  scale.  The  Gay-Lussac  hydrom- 
eter is  therefore  usually  made  in  the  form  represented  in  Fig.  28. 
For  the  purpose  of  effecting  its  graduation,  the  instru- 
ment is  placed  first  in  water  of  the  standard  temperature 
and  then  in  sulphuric  acid  of  the  same  temperature 
which  has  a  specific  gravity  of  1.25.  The  points  to 
which  it  sinks  in  the  two  liquids  are  designated  by  the 
numbers  100  and  80  respectively.  The  space  between 
these  two  fixed  points  is  graduated  into  20  equal  parts, 
and  the  graduation  is  then  continued  uniformly  up  and 
down  the  tube.  The  specific  gravity  of  any  liquid  will 
be  found  by  dividing  100  by  the  number  of  volumes 
immersed,  as  read  upon  the  scale  of  the  instrument. 

The  volumeter  may  also  be  graduated  without  the 
aid  of  a  second  liquid  of  known  specific  gravity.     The 
FIG  28     method  is  as  follows :  The  instrument  is  weighted  with 
mercury  until  it  sinks  in  water  to  the  point  where  it  is 
desired  to  place  the  1 00  mark ;  its  total  weight  is  then  exactly 
doubled  by  the  introduction  of  more  mercury,  and  the  point  to 
which  it  sinks  is  designated  by  the  number  200.     The  space 


140  QUANTITATIVE  EXERCISES 

between  the  100  and  the  200  marks  is  divided  into  100  equal 
parts,  and  the  graduation  is  continued  uniformly  down  the  tube 
as  far  as  may  be  desired.  Instead  of  doubling  the  weight  of 
the  instrument  for  the  purpose  of  establishing  a  second  point 
in  the  scale,  its  weight  may  be  reduced  to  one-half  by  removing 
mercury.  The  point  to  which  the  volumeter  then  sinks  in  water 
would  be  designated  by  50. 

The  hydrometers  of  Balling  and  of  Brix  are  constructed  on 
the  same  principle  as  the  volumeter  of  Gay-Lussac,  the  only 
difference  between  them  and  the  latter  being  in  the  magnitude 
of  the  "degree."  In  the  instrument  of  Balling  the  point  to 
which  the  hydrometer  sinks  in  water  is  designated  by  200,  and 
in  that  of  Brix  by  400.  A  degree  Gay-Lussac  is  therefore  equal 
to  2  degrees  Balling  and  4  degrees  Brix. 

2.  SPECIFIC-GRAVITY  HYDROMETERS 

Certain  hydrometers  of  variable  immersion  are  graduated  by 
placing  the  instruments  in  two  or  more  liquids  of  different 
known  densities,  and  then  dividing  the  spaces  between  the 
points  thus  established,  also  the  spaces  above  and  below  them, 
into  equal  divisions  whose  length  bears  the  proper  rela- 
tion to  the  differences  between  the  specific  gravities  of 
the  liquids.  That  such  instruments  do  not  in  reality 
record  the  densities  of  liquids  except  at  the  points 
experimentally  established  will  appear  from  the  follow- 
ing  consideration.  Suppose  we  have  six  different  liquids 
whose  densities  are  related  as  the  whole  numbers  from 
1  to  6  ;  also  that  we  have  a  glass  tube,  Fig.  29,  of  uni- 
form diameter,  closed  at  both  ends,  and  of  such  weight 
and  volume  that  it  will  sink  in  liquid  No.  1  until  its  upper  end  is 
on  a  level  with  the  surface.  Let  the  whole  length  of  the  tube 
be  represented  by  60.  If  the  tube  is  placed  in  liquids  Nos.  2, 
3,  4,  5,  and  6  in  succession,  the  points  marked  30,  20,  15,  12, 
and  10  will,  each  in  turn,  coincide  with  the  upper  surface  of  a 


THE    DETERMINATION  OF   SPECIFIC   GRAVITY        141 

liquid;  for,  according  to  the  law  that  a  body  will  sink  in  a 
liquid  until  it  has  displaced  its  own  weight,  one-half  of  the 
tube  will  be  submerged  in  liquid  No.  2,  one-third  in  No.  3, 
one  fourth  in  No.  4,  one-fifth  in  No.  5,  and  one-sixth  in  No.  6. 
In  other  words,  while  the  densities  of  the  liquids  are  related 
to  each  other  as  1,  2,  3,  4,  etc.,  the  volumes  immersed  are 
related  as  the  reciprocals  of  these  numbers.  Accordingly,  it 
will  be  noticed  on  the  graduation  of  specific-gravity  hydrom- 
eters that  the  divisions  corresponding  to  equal  differences  of 
density  diminish  in  length  in  a  downward  direction.  Such 
instruments  are  said  to  be  harmonically  graduated. 

3.  BEAUME'S  HYDROMETER 

This  instrument  is  much  used  in  commercial  transactions 
both  on  the  continent  of  Europe  and  in  America.  It  consists 
of  two  spindles,  one  for  liquids  lighter  and  another  for  liquids 
heavier  than  water.  A  spindle  which  is  to  be  graduated  for 
liquids  lighter  than  water  is  placed  in  a  10  per  cent  solution 
of  sodium  chloride.  The  point  to  which  it  sinks  is  designated 
by  0.  It  is  then  transferred  to  water  and  the  point  to  which  it 
sinks  in  this  liquid  is  designated  by  10.  The  space  between 
these  two  points  is  divided  into  ten  equal  parts,  and  the  gradua- 
tion is  continued  uniformly  up  the  stem.  A  spindle  which  is 
to  be  graduated  for  liquids  heavier  than  water  is  placed  first  in 
water  and  then  in  a  10  per  cent  solution  of  sodium  chloride 

15° 

(sp.  gr.  -—j  =  1.07335).     The  points  thus  established  are  des- 
lo 

ignated  by  0  and  10  respectively.  The  space  between  them  is 
divided  into  ten  equal  parts,  and  the  graduation  is  continued 
uniformly  down  the  stem. 

It  will  be  seen  that  the  so-called  degrees  Beaume  stand  in 
no  very  simple  relation  to  the  specific  gravities  of  liquids. 
Expressed  in  density,  the  degree  66  on  the  instrument  for 
heavy  liquids  has  somewhat  over  three  times  the  value  of 


142  QUANTITATIVE  EXERCISES 

degree  1.  Considerable  confusion  has  arisen  from  the  negli- 
gence of  the  early  makers  both  in  determining  and  in  stating 
the  exact  temperature  and  density  of  the  liquids  which  they 
employed  in  graduating  the  instruments.  The  earlier  hydrom- 
eters were  graduated  for  12.5°,  15°,  and  17.5°.  At  present, 
there  is  in  use  for  liquids  heavier  than  water  a  Beau  me  hydrom- 
eter with  a  so-called  "rational  scale."  This  is  graduated  by 
placing  the  spindle  first  in  water  at  15°  and  then  in  sulphuric 

15° 

acid  of  specific  gravity  1.842  (d  —  =1.842).     The  points  to 

lo 

which  it  sinks  are  designated  by  0  and  66  respectively,  and  the 
space  between  them  is  divided  into  66  equal  parts. 

The  following  formulas  have  been  worked  out  for  finding 
the  specific  gravity  corresponding  to  any  degree  Beaume. 
The  letter  n  signifies  the  number  of  the  degree  Beaume,  and 
t  the  temperature  for  which  the  instrument  is  graduated. 

Instruments  for  Liquids  Lighter  than  Water 

145.88 


=  12.5°,  d  = 


135.88  +  n 

146.3 
136.3  +  n* 

146.78 
136.78  +  n' 


Instruments  for  Liquids  Heavier  than  Water 

145.88 


£  =  12.5°,  d  = 
t  =  15°,      d  = 


145.88  -  n 

146.3 
146.3  -n 

146.78 
146.78 -n' 


THE  DETERMINATION  OF  SPECIFIC  GRAVITY        143 

Instrument  with  "  Rational  Scale  " 
144.3 


t  = 


144.3  -  n 


The  "  Dutch  hydrometer  "  is  the  older  Beau  me.  It  is  grad- 
uated for  12.5°,  and  the  10  per  cent  salt  solution  employed  in 
its  graduation  is  assumed  to  have  a  specific  gravity  of  1.074626 

15° 

(d  — ).     The  formulas  for  finding  the  corresponding  specific 
15 

gravities  are 

144 
d  =  :— for  liquids  lighter  than  water,  and 

144 

d  =  -r-r- for  liquids  heavier  than  water. 

144  —  n 


4.  BECK'S  HYDROMETER 

This  instrument  is  graduated  by  placing  the  spindle  in  water 

12  5° 

and  then  in  a  liquid  having  a  specific  gravity  of  0.85  (d      '     )• 

l^j.5 

The  points  to  which  it  sinks  are  designated  by  0  and  30  respec- 
tively. The  space  between  them  is  divided  into  30  equal  parts, 
and  the  graduation  is  continued  uniformly  both  up  and  down 
the  stem.  The  standard  temperature  is  12.5°.  The  formulas  for 
finding  the  specific  gravity  corresponding  to  any  degree  Beck  are 

170 
d  =  — — for  liquids  lighter  than  water,  and 

170 

d  —  - for  liquids  heavier  than  water. 

170  —  n 


5.  TWADDELL'S  HYDROMETER 

Twaddell's  hydrometer,  which  is  much  used  in  England,  is 
designed  for  liquids  heavier  than  water.  The  interval  between 
the  points  to  which  it  sinks  in  water  and  in  a  liquid  twice  as 


144  QUANTITATIVE  EXERCISES 

heavy  is  harmonically  divided  into  200  spaces  or  degrees,  so 
that  successive  divisions  on  the  scale  correspond  to  an  increase 
or  decrease  of  0.005  in  specific  gravity.  Hence  the  specific 
gravity  corresponding  to  any  degree  Twaddell  may  be  found 
by  multiplying  the  number  of  degrees  by  0.005  and  adding  the 
product  to  1.000.  Thus  a  liquid  having  a  density  of  10  degrees 
Twaddell  has  a  specific  gravity  of  10  x  0.005  +  1.000,  or  1.05. 
The  Twaddell  instrument  usually  consists  of  five  separate 
spindles. 

6.  THE  ALCOHOLOMETER 

There  are  also  in  use  a  number  of  special  hydrometers,  i.e. 
instruments  which  serve  to  determine  the  concentration,  and 
consequently  the  commercial  value,  of  individual  solutions. 
The  most  generally  useful  of  these  is  the  alcoholometer.  There 
are  two  kinds  of  alcoholometers.  One  gives  the  percentage  of 
alcohol  in  an  aqueous  solution  by  volume,  and  the  other  by 
weight.  The  latter  kind  is  but  little  used,  either  in  the  reve- 
nue service  or  in  commercial  transactions.  Alcoholometers  are 
graduated  for  a  particular  temperature  which  is  designated  upon 
the  instruments.  But  each  instrument  is  accompanied  by  a 
table  in  which  are  to  be  found  the  necessary  corrections  when 
the  solutions  have  other  than  the  standard  temperature. 

7.  THE  LACTOMETER 

The  lactometer  is  a  very  sensitive  specific-gravity  spindle 
with  a  scale  ranging  from  perhaps  1.01  to  1.04.  The  com- 
monest form  of  the  instrument  is  the  "  lactodensimeter "  of 
Quevenne,  which  is  employed  to  detect  and  to  estimate  ap- 
proximately the  falsification  of  milk,  either  by  diluting  with 
water  or  by  skimming.  The  specific  gravity  of  normal  milk 
ranges  from  1.026  to  1.032.  If  any  considerable  portion  of  the 
butter  is  removed,  the  density  of  the  milk  becomes  too  great; 
while  if  much  water  is  added,  it  falls  below  the  normal  limit. 


THE  DETERMINATION  OF  SPECIFIC   GRAVITY        145 

The  scale  of  the  lactodensimeter  ranges  from  1.014  to  1.042,  and 
the  readings  are  grouped  together  by  a  series  of  braces  placed 
to  the  right.  The  first  of  these  includes  the  probable  range  of 
density  of  pure  milk ;  the  second,  that  of  milk  diluted  with  one- 
tenth  of  its  volume  of  water ;  etc.  In  some  countries  there  is 
also  legalized,  under  proper  representations  as  to  its  character, 
the  sale  of  so-called  "  half-skimmed  "  milk.  This  milk  is  also 
required  to  come  up  to  certain  standards  as  to  percentage  of 
butter  and  other  nutritious  constituents.  There  is  therefore 
placed  to  the  left  of  the  Quevenne  scale  a  second  grouping  of 
readings  which  serves  to  detect  excessive  skimming  or  dilution 
with  water. 

SPECIFIC-GRAVITY  BULBS 

These  are  hollow  glass  bulbs  on  each  of  which  is  recorded 
the  specific  gravity  of  a  liquid  in  which  it  will  neither  sink  nor 
rise.  They  are  made  by  blowing  bulbs  on  the  end  of  a  glass 
tube.  The  bulbs  are  first  roughly  assorted  by  throwing  them 
into  liquids  of  different  specific  gravities  and  afterwards  they 
are  accurately  adjusted  by  grinding. 

DETERMINATION  OF  SPECIFIC  GRAVITY  BY  DILUTION 

The  specific  gravity  of  a  solid  may  be  determined  by  placing 
it  in  a  liquid  more  dense  than  itself  and  then  diluting  with  a 
lighter  liquid  until  the  body  will  neither  float  nor  sink.  At 
this  stage  the  solid  and  the  liquid  are  of  the  same  density,  and 
the  specific  gravity  of  the  former  may  be  ascertained  by  deter- 
mining that  of  the  latter  by  any  of  the  methods  applicable  to 
liquids.  This  procedure  is  resorted  to  with  advantage  when 
the  quantity  of  available  material  is  too  small  to  admit  of  an 
accurate  determination  of  its  specific  gravity  by  other  methods. 

Saturated  solutions  of  the  double  iodide  of  potassium  and 
mercury  (sp.  gr.  3.196),  of  cadmium  borotungstate  (sp.  gr.  3.36), 
and  of  the  double  iodide  of  barium  and  mercury  (sp.  gr.  3.588), 


146  QUANTITATIVE  EXERCISES 

also  methylene  iodide  (sp.  gr.  3.324),  are  employed  in  this  man- 
ner, not  only  for  the  determination  of  specific  gravity,  but  also 
for  the  separation  of  minerals  in  rock  mixtures.  The  double 
iodide  of  potassium  and  mercury  (Thoulet's  solution)  and  the 
borotungstate  of  cadmium  (Klein's  solution)  may  be  diluted  with 
water,  but  not  the  double  iodide  of  barium  and  mercury  (Rohr- 
bach's  solution),  which  deposits  mercuric  iodide  when  water  is 
added.  Methylene  iodide  (Braun's  liquid)  may  be  diluted  with 
benzene  or  with  toluene.  The  last  liquid,  methylene  iodide,  is 
used  for  the  determination  of  the  specific  gravity  and  the  sepa- 
ration of  substances  which  are  soluble  in  water. 


THE  DENSITY  OF  GASES 

The  exact  determination  of  the  relative  densities  of  the  dif- 
ferent gases  is  one  of  the  most  important  and  at  the  same  time 
the  most  difficult  of  all  quantitative  chemical  problems.  The 
problem  is  more  difficult  in  the  case  of  gases  than  in  that  of 
liquids  and  solids,  because  of  the  greater  number,  magnitude, 
and  complexity  of  the  sources  of  error.  Some  of  the  obstacles 
to  a  satisfactory  solution  of  it  will  be  mentioned. 

Owing  to  its  universal  presence  as  a  constituent  of  the  air, 
and  the  difficulty  of  removing  it  by  absorption,  it  is  not  easy  to 
obtain  any  gas  which  is  wholly  free  from  nitrogen. 

The  prevalence  of  water  vapor  and  the  difficulty  of  wholly 
removing  it  from  gases  is  another  source  of  embarrassment. 

The  rapid  diffusion  of  gases  is  always  a  source  of  danger  to 
their  purity.  It  makes  their  isolation  difficult,  and  renders  the 
complete  displacement  of  one  gas  by  another  impracticable 
except  through  the  intervention  of  the  vacuum  pump  with  its 
complex  manipulation. 

The  great  influence  of  temperature  and  pressure  upon  the 
densities  of  gases  makes  necessary  exceptional  precautions  in 
the  determination  of  the  temperature  and  pressure  under  which 
they  are  measured ;  while  the  variability  of  the  standard  for 


THE  DETERMINATION  OF  SPECIFIC   GRAVITY        147 

pressure  and  the  lack  of  a  rigid  obedience  on  the  part  of  gases 
to  the  laws  of  Boyle  and  of  Gay-Lussac  are  a  source  of  some 
uncertainty  in  the  calculations  which  are  based  on  such  deter- 
minations. 

Finally,  the  accuracy  of  the  results  is  diminished  by  the  fact 
that  it  is  impossible  to  operate  with  more  than  very  small 
weights  of  gases  and  that  even  these  must  be  weighed  in  con- 
taining vessels  which  are  many  times  heavier  than  the  gases 
themselves. 

The  densities  of  those  gases  which  were  formerly  designated 
as  the  permanent  gases  have  been  determined  in  various  ways. 
Of  the  methods  employed  the  following  are  the  more  important. 

1.  BY  WEIGHING  THE  GASES  DIRECTLY 

To  determine  the  density  of  gases  by  weighing,  two  large 
balloon  flasks  of  equal  volume  and  weight,  and  provided  with 
gas-tight  stopcocks,  are  suspended  from  the  arms  of  a  sensitive 
balance.  One  of  them  is  closed  and  serves  only  as  a  counter- 
poise, while  the  other  is  used  as  the  containing  vessel  for  the 
gases  to  be  weighed.  The  weight  of  the  latter  is  found  by 
weighing  the  exhausted  vessel,  and  its  capacity  by  weighing 
the  water  which  is  required  to  fill  it.  It  is  then  filled  with  the 
gas  under  investigation  and  again  weighed.  The  use  of  a  coun- 
terpoise .of  the  same  material  and  volume  as  the  containing  ves- 
sel is  important  not  only  as  a  correction  for  air  displacement 
but  also  as  a  means  of  counteracting  the  errors  in  weighing 
which  arise  from  the  variable  condensation  of  atmospheric 
moisture  on  the  surface  of  bodies. 

Lord  Rayleigh  has  recently  pointed  out  that  this  method 
(which  was  employed  by  Regnault  and  afterwards  by  many 
others)  has  in  it  a  source  of  error  hitherto  unsuspected, 
namely,  the  shrinkage  of  the  containing  vessel  when  it  is 
exhausted  for  the  purpose  of  determining  its  weight.  The 
result  of  such  a  shrinkage  would  be,  of  course,  an  incorrect 


148 


QUANTITATIVE  EXERCISES 


determination  of  the  weight,  first  of  the  vessel  and  ultimately 
of  the  gases  themselves.  J.  M.  Crafts  has  determined  experi- 
mentally the  probable  amount  of  this  contraction  in  the  vessel 
used  by  Regnault,  and  therefore  the  correction  which  is  to  be 
applied  to  Regnault's  results.  He  employed  for  the  purpose  not 
the  vessel  which  Regnault  had  used,  for  that  had  been  destroyed, 
but  one  similar  in  all  respects,  which  had  been  made  at  the  same 
factory  and  at  the  same  time.  The  shrinkage  was  found  by 
Crafts  to  amount  to  0.000247  of  the  volume  of  the  vessel. 

The  following  table  contains  the  corrected  results  of  Reg- 
nault for  the  latitude  of  Paris. 


DENSITY 

(Air  =  l) 

DENSITY 

(Hydrogen  —  1) 

WEIGHT  OF  1  LITER 

(at  Paris) 

Air 

1.00000 

14.391 

1.29349  grms. 

Nitrogen 

0.97138 

13.979 

1.25647      " 

Hydrogen 

0.06949 

1.000 

0.08  98'8      « 

Oxygen 

1.10562 

15.910 

1.43011      " 

Carbon  Dioxide 

1.52897 

22.003 

1.97772      « 

2.  BY  WEIGHING  THE  GASES  AFTER  ABSORPTION 

This  method  was  first  employed  by  R.  F.  Marchand.  A 
large  glass  vessel  was  filled  at  0°  and  under  a  constant  pres- 
sure (765  mm.)  with  a  pure  gas.  The  gas  was  then  forced,  by 
the  introduction  of  a  neutral  gas,  through  a  weighed  tube  con- 
taining the  appropriate  absorbent.  The  increase  in  the  weight 
of  the  tube  gave  the  weight  of  the  gas.  In  this  way  Marchand 
determined  the  relative  densities  of 

Oxygen,  which  was  displaced  by  carbon  dioxide  and  absorbed 
by  red-hot  copper ; 

Carbon  dioxide,  which  was  displaced  by  air  and  absorbed  by 
potassium  hydroxide ; 

Carbon  monoxide,  which  was  burned  by  copper  oxide  and  the 
resulting  carbon  dioxide  absorbed  in  potassium  hydroxide ; 


THE  DETERMINATION  OF  SPECIFIC   GRAVITY        149 

Sulphur  dioxide,  which  was  displaced  by  hydrogen  and 
absorbed  by  potassium  hydroxide. 

Since  by  this  method  the  vessel  is  always  filled  under  the 
same  conditions,  the  volumes  of  the  several  gases  are  equal  and 
the  weights  found  are  therefore  strictly  proportional  to  their 
densities.  It  will  be  seen  that  it  is  not  necessary  to  know  the 
capacity  of  the  vessel  unless  in  addition  to  their  relative  densi- 
ties it  is  desired  to  ascertain  the  weights  of  a  unit  volume  of 
the  gases. 

In  one  respect  the  method  of  Marchand  is  superior  to  that 
of  Regnault.  It  does  away  with  the  necessity  of  weighing  the 
containing  vessel,  which  is  the  most  critical  step  in  the  Reg- 
nault procedure. 

A  similar  method  was  employed  by  J.  P.  Cooke  to  ascertain 
the  weight  of  the  containing  vessel  which  he  used  in  his  deter- 
mination of  the  atomic  weight  of  oxygen.  The  vessel  was 
filled  with  carbon  dioxide  and  weighed.  The  gas  was  then 
absorbed  and  weighed.  The  difference  between  the  two  weights 
was  the  weight  of  the  vessel. 

It  will  be  seen  that  the  displacement  and  absorption  method 
may  also  be  used  to  determine  the  capacity  of  a  vessel,  or  an 
apparatus,  such,  for  instance,  as  an  air  pump. 


3.  BY  THE  TIME  REQUIRED  FOR  DIFFUSION 

It  was  shown  by  Graham  that,  under  the  same  conditions 
of  temperature  and  pressure,  the  times  required  for  the  diffu- 
sion through  a  small  aperture  of  equal  volumes  of  different 
gases  are  inversely  proportional  to  the  square  roots  of  the  densi- 
ties of  the  gases.  Bunsen  (see  Grasometrische  Methoden,  2.  Auflage, 
page  184)  has  founded  upon  this  principle  a  simple  method 
for  the  determination  of  the  relative  densities  of  gases.  The 
method  is  not  more  accurate  than  that  of  Regnault  or  of 
Marchand,  but  has  the  advantage  of  requiring  only  small  vol- 
umes of  the  gases. 


CHAPTER  VII 

THE  DETERMINATION  OF  MOLECULAR  WEIGHTS 
ATOMIC  RATIOS 

The  following  examples  are  given  to  illustrate  the  method  by 
which  one  finds,  from  the  results  of  a  quantitative  analysis  of 
any  compound,  the  atomic  ratios  of  the  constituents. 

1.  COPPER  OXIDE 

Suppose  by  reducing  a  known  weight  of  cupric  oxide  in  a 
current  of  hydrogen  and  weighing  the  resulting  metallic  copper, 
it  is  found  that  the  compound  contains  79.8  per  cent  of  copper 
and  20.1  per  cent  of  oxygen.  If  we  divide  the  first  quantity  by 
63.1,  the  atomic  weight  of  copper,  and  the  second  by  15.88,  the 
atomic  weight  of  oxygen,  we  shall  obtain  the  numbers  1.264  and 
1.265,  which  are  related  to  each  other  as  are  the  numbers  of  the 
atoms  of  the  two  elements  in  the  compound.  If  we  divide  the 
numbers  representing  the  atomic  ratios  by  the  smaller  of  the  two 
(1.264),  with  a  view  to  reducing  them  to  whole  numbers  with- 
out disturbing  their  ratio,  we  shall  obtain  1  and  1.00079.  The 
second  number  is  so  nearly  unity  that  we  may  fairly  assume 
that  the  excess,  0.00079,  originated  in  the  unavoidable  errors  of 
analysis  or  is  due  to  a  slight  inaccuracy  in  the  number  repre- 
senting the  atomic  weight  of  copper.  If  so,  there  are  as  many 
atoms  of  copper  as  of  oxygen  in  a  molecule  of  copper  oxide, 
and  the  simplest  formula  which  we  can  assign  to  the  compound 
is  CuO. 

150 


DETERMINATION  OF  MOLECULAR  WEIGHTS         151 

2.  POTASSIUM  PERMANGANATE 

An  analysis  of  this  compound  shows  it  to  consist,  approxi- 
mately, of  potassium  24.7  per  cent,  manganese  34.8  per  cent, 
and  oxygen  40.5  per  cent.  By  dividing  the  percentage  of  each 
constituent  by  its  atomic  weight  and  reducing  the  ratio  to  one 
of  whole  numbers,  we  find  that  K  :  Mn  :  O  : :  1  : 1  :  4.  That  is, 
there  are  in  the  compound  as  many  atoms  of  potassium  as  of 
manganese  and  four  times  as  many  atoms  of  oxygen  as  of  either 
potassium  or  manganese.  The  simplest  possible  formula  for  the 
compound  is  therefore  KMnO4. 

3.  ACETIC  ACID 

Acetic  acid  contains  very  nearly  40  per  cent  of  carbon,  53.3 
per  cent  of  oxygen,  and  6.7  per  cent  of  hydrogen.  Hence 
C  :  O  :  H  : :  3.361  :  3.356  :  6.700  : :  1.001  :  1  :  1.996.  The  slight 
variations  from  whole  numbers  are  probably  due  to  errors  of 
analysis  and  the  slight  inaccuracies  of  the  numbers  assumed  to 
represent  the  atomic  weights  of  carbon  and  hydrogen.  We  must 
therefore  conclude  that  acetic  acid  contains  as  many  atoms  of 
carbon  as  of  oxygen  and  twice  as  many  atoms  of  hydrogen  as 
of  either  carbon  or  oxygen.  The  simplest  formula  which  the 
compound  can  have  is  CH2O. 

4.  PROPYL-AMINE 

This  compound  contains  63  per  cent  of  carbon,  23.75  per 
cent  of  nitrogen,  and  15.25  per  cent  of  hydrogen.  Proceeding 
as  before,  we  find  that 

C  :  H  :  N  : :  5.294  :  15.25  :  1.7049  : :  3.105  :  8.989  :  1  : :  3  :  9  : 1. 
As  determined  by  analysis,  the  formula  of  the  compound  is 
therefore  C3H9N. 


152  QUANTITATIVE  EXERCISES 

5.  BENZENE 

Benzene  contains  about  92.3  per  cent  of  carbon  and  7.7  per  cent 
of  hydrogen.  Dividing  these  numbers  by  11.9  and  1  respectively, 
we  obtain  7.756  and  7.7.  Hence  C  :  H  : :  7.756  : 7.7  : :  1 : 1,  and  the 
simplest  formula  which  can  be  assigned  to  the  compound  is  CH. 

In  the  examples  previously  given  it  has  been  necessary,  in 
order  to  find  the  atomic  ratios  of  the  constituents  of  a  compound, 
only  to  divide  the  percentage  -of  each  element  by  its  atomic 
weight  and  then  to  divide  all  of  the  quotients  by  the  smallest 
one  among  them.  The  results  were  in  all  cases  so  nearly  whole 
numbers  that  the  fractions  could  reasonably  be  ascribed  to  errors 
of  analysis  or  to  the  use  of  slightly  incorrect  values  for  the 
atomic  weights  of  the  elements.  These  fractions  are,  however, 
often  too  large  to  be  accounted  for  in  this  manner.  In  such 
cases  it  is  clear  that  the  ratios  must  be  multiplied  by  numbers 
which  will  convert  the  fractions  into  whole  numbers.  To  illus- 
trate this,  we  will  consider  the  case  of  toluene.  The  compound 
is  shown  by  analysis  to  contain  91.3  per  cent  of  carbon  and 
8.7  per  cent  of  hydrogen.  Proceeding  as  before,  we  find  that 
C  :  H  : :  7.672  :  8.7  : :  1  : 1.1339.  The  fraction  0.1339  amounts 
to  11.8  per  cent,  or  about  one-eighth  of  the  whole  hydrogen. 
That  an  error  of  this  magnitude  could  have  been  made  in 
the  analysis  is  improbable.  We  must  therefore  conclude  that 
C  :  H  : :  1  X  7  :  1.1339  x  7  : :  7  :  7.9373  : :  7  :  8,  and  that  the  sim- 
plest formula  which  toluene  can  have  is  C7H8. 

The  determination  of  atomic  ratios  in  compounds  of  high 
molecular  weights  is  somewhat  uncertain  owing  to  the  consider- 
able effect,  in  such  cases,  of  small  errors  of  analysis.  Suppose  a 
certain  hydrocarbon  to  contain,  theoretically,  85.05  per  cent  of 
carbon  and  14.95  per  cent  of  hydrogen.  Using  11.9  and  1  as 
the  atomic  weights  of  the  elements,  we  should  find  that 

C  :  H  : :  7.14706  : 14.95  : :  1 :  2.091825  : :  10  :  20.92. 
Regarding  the  fraction  0.92  as  due  to  the  fact  that  the  calcula- 
tion of  the  percentages  of  the  constituents  is  correct  only  to  the 


DETERMINATION  OF  MOLECULAR  WEIGHTS         153 

second  decimal  place,  we  should  conclude  that  the  atomic  ratio 
of  the  elements  is  expressed  by  the  formula  C10H21.  Suppose, 
however,  on  analyzing  the  compound  for  the  purpose  of  deter- 
mining the  ratio,  we  obtained  84.85  per  cent  of  carbon  and  15.15 
per  cent  of  hydrogen,  i.e.  0.2  per  cent  too  little  of  the  former 
and  0.2  per  cent  too  much  of  the  latter.  We  should  then  find 
that  C  :  H  : :  7.13025  :  15.15  : :  1  :  2.12475  : :  7  :  14.873  : :  7  :  15, 
and  assign  to  the  compound  the  formula  C7H15. 

We  are  able,  by  means  of  the  foregoing  process,  to  ascertain 
the  relative  but  not  the  absolute  numbers  of  the  different  kinds 
of  atoms  in  a  compound  molecule.  It  is  possible,  for  instance, 
to  determine  by  analysis  that  in  a  molecule  of  acetic  acid 
C:H:O::1:2:1.  It  does  not  follow,  however,  that  the 
formula  of  the  compound  is  CH2O.  It  may  be  C2H4O2,  C3H6O3, 
or  any  other  multiple  of  the  ratio  Cl :  H2  :  Or  To  decide  which 
of  these  possible  formulas  is  the  correct  one,  the  chemical 
analysis  must  be  supplemented  by  a  determination  of  the 
molecular  weight  of  the  substance. 


MOLECULAR  WEIGHTS 

The  methods  by  which  molecular  weights  are  determined  are 
usually  classified  as  chemical  and  physical. 

THE  CHEMICAL  METHOD 

The  chemical  method  of  determining  molecular  weights  can- 
not be  concisely  defined.  It  consists,  in  general,  of  a  study  of 
the  reactions  of  the  substances,  both  those  which  can  be  followed 
quantitatively  and  those  which  throw  light  upon  the  character 
of  the  groupings  of  the  atoms  within  the  molecules.  If  it  can 
be  shown,  for  example,  that  a  given  compound  contains  two 
hydroxyl  groups,  or  one  hydroxyl  and  one  carbonyl  group,  it  is 
certain  that  the  compound  contains  not  less  than  two  atoms  of 
oxygen.  Again,  if  its  hydrogen  can  be  replaced  by  another 


154  QUANTITATIVE   EXERCISES 

univalent  element  or  group  in  four  different  but  equal  portions, 
it  is  certain  that  the  compound  contains  four,  or  some  multiple 
of  four,  atoms  of  hydrogen.  And  if  never'  less  than  one-fourth 
of  the  hydrogen  is  replaced  at  any  one  time,  it  is  probable  that 
the  compound  contains  exactly  four,  and  not  some  multiple  of 
four,  atoms  of  hydrogen.  Many  formulas,  otherwise  plausible, 
may  be  discarded  for  the  simple  reason  that  they  are  incon- 
sistent with  the  known  valence  of  the  constituents;  e.g.  the 
formula  CH  for  benzene  would  be  rejected  because  it  repre- 
sents carbon  and  hydrogen  as  of  equal  valence. 

The  usefulness  and  some  of  the  limitations  of  the  chemical 
method  will  be  illustrated  by  applying  it  to  the  compounds 
already  cited. 

The  atomic  ratio  of  copper  and  oxygen  in  cupric  oxide  was 
found  by  analysis  to  be  as  1  : 1.  The  simplest  possible  formula 
of  the  substance  is  therefore  CuO.  There  is  no  reason  for 
rejecting  this  formula  as  inconsistent  with  what  is  believed  to  be 
the  valence  of  the  constituents,  nor  is  any  evidence  of  a  more 
complicated  composition  to  be  found  in  any  of  its  reactions. 
We  therefore  accept  the  formula  CuO  as  probably  correct. 

In  the  case  of  potassium  permanganate  the  chemical  evidence 
is  nearly  as  negative  as  in  that  of  copper  oxide.  In  fact,  very 
little  evidence  can  be  obtained  from  the  reactions  of  the  com- 
pound, either  as  to  the  relations  of  the  atoms  within  the  mole- 
cule or  as  to  their  absolute  number.  It  is  therefore  impossible 
to  decide  by  means  of  the  chemical  method  whether  the  formula 
of  the  compound  is  KMnO4,  K2Mn2O8,  K3Mn3O12,  or  some 
other  multiple  of  KjMnjO^ 

The  method  may,  however,  be  applied  to  acetic  acid  with 
more  satisfactory  results.  It  is  shown  by  analysis  that  in  this 
compound  C  :  H  :  O  ::  1 :  2  : 1,  and  the  simple  formula  CH2O  is 
in  accord  with  the  supposed  quadrivalence  of  carbon,  the  biva- 
lence  of  oxygen,  and  the  univalence  of  hydrogen.  But  a  study 
of  the  reactions  of  the  substance  brings  to  light  many  reasons 
for  assigning  to  it  the  formula  C2H4O2.  Two  of  these  will  be 


DETERMINATION  OF  MOLECULAR  WEIGHTS          155 

given.  There  is,  on  the  whole,  good  reason  for  regarding  acetic 
acid  as  mono-basic,  but  when  its  salts  with  metals  are  examined 
they  are  found  still  to  contain  three-fourths  of  the  hydrogen  of 
the  acid,  proving  the  presence  of  4  atoms  of  hydrogen  in  the 
acid.  Again,  one,  two,  and  three  fourths  of  the  hydrogen  of 
the  acid  may  be  successively  replaced  by  one,  two,  and  three 
equivalents  of  chlorine.  The  molecular  weights  of  acids  may 
be  determined  by  an  analysis  of  their  salts.  As  already  stated, 
acetic  acid  is  regarded  as  mono-basic.  If  this  view  is  correct,  its 
salts  are  formed  by  the  replacement  of  a  single  atom  of  hydro- 
gen in  each  molecule  of  the  acid  by  a  metal  or  its  equivalent, 
and  its  salts  containing  univalent  metals  consist  of  one  acid 
residue  (a  molecule  of  the  acid  less  one  atom  of  hydrogen) 
and  a  single  atom  of  metal.  Evidently  a  determination  of  the 
quantity  of  metal  which  is  contained  in  a  known  weight  of  any 
such  salt  will  enable  us  to  ascertain  the  molecular  weight  of 
the  acid.  The  silver  salts  of  acids  are  usually  employed  in  such 
determinations  of  molecular  weights.  They  are  easily  obtained 
in  pure  condition,  and  their  analysis  is  exceedingly  simple.  If 
a  weighed  quantity  of  silver  acetate  is  ignited  in  a  crucible,  the 
acid  residue  is  volatilized  and  there  remains  in  the  crucible 
nothing  but  the  silver.  On  weighing  this  it  is  found  that  the 
salt  consists  of  very  nearly  64.67  per  cent  of  silver  and  35.33 
per  cent  of  volatile  matter.  Hence  64.67  :  35.33  ::  107.11 :  the 
molecular  weight  of  the  acid  residue.  The  proportion  gives  us 
58.51  as  the  weight  of  the  acid  which  was  in  combination  with 
107.11  parts  (one  atom)  of  silver.  If  we  add  to  this  1  for  the 
atom  of  hydrogen  which  was  displaced  by  the  silver,  we  shall 
obtain  59.51  as  the  molecular  weight  of  acetic  acid,  which  cor- 
responds to  the  formula  C2H4O2  (23.8  +4  -f- 31.76  =  59.56). 
This  method  of  determining  molecular  weights  is  applicable  to 
all  mono-basic  acids.  It  may  also  be  applied  to  poly-basic  acids 
provided  it  is  known  with  sufficient  certainty  what  proportion 
of  the  acid  hydrogen  has  been  replaced  in  forming  the  salts 
which  are  analyzed. 


156  QUANTITATIVE  KXKKCISKS 

The  molecular  weights  of  many  organic  bases  may  be  deter- 
mined by  a  method  as  simple  as  that  which  is  employed  for  the 
acids.  These  bodies  are  to  be  regarded  as  substituted  ammo- 
nias, and,  like  ammonia,  they  form  well-defined  double  salts 
when  treated  with  platinum  chloride  and  hydrochloric  acid. 
Sucli  salts  are  decomposed,  when  heated,  with  volatilization  of 
everything  except  the  platinum.  If  a  weighed  quantity  of  the 
double  chloride  of  propyl-amine  and  platinum  is  heated  in  a 
crucible,  the  salt  is  found  to  consist  of  37  per  cent  of  platinum 
and  68  per  cent  of  volatile  matter.  Hence  37  :  63  ::  193.4  (at. 
wt.  Pt.) :  molecular  weight  of  the  volatile  matter.  The  propor- 
tion gives  329.3  as  the  weight  of  all  the  matter  which  was  in 
combination  with  193.4  parts,  or  one  atom,  of  platinum.  The 
molecular  weight  of  the  double  salt  is  therefore  193.4  +  329.3, 
or  522.7.  We  may  regard  a  molecule  of  the  double  chloride 
of  ammonium  and  platinum  as  made  up  of  two  molecules  ,of 
ammonia,  two  of  hydrochloric  acid,  and  one  molecule  of  plati- 
num chloride;  thus  (NH8)2  (HCl)aPtCl4.  The  corresponding 
double  salt  of  propyl-amine  would  then  be  (propyl-amine)2 
(HCl)2PtCl4;  and  if  we  subtract  from  522.7  (the  molecular 
weight  of  the  compound,  406.48)  the  molecular  weight  of 
(HCl)2PtCl4,  the  difference,  116.22,  will  be  the  weight  of  the 
two  molecules  of  the  base.  The  molecular  weight  of  propyl- 
amine  is  therefore  58.68,  and  its  formula  is  C8H9N  (35.7+  9  + 
18.98  =  58.63). 

This  method  suffices  for  the  determination  of  the  molecular 
weights  of  all  mono-acid  bases,  i.e.  for  any  base  a  molecule  of 
which  is  derived  by  substitution  from  a  single  molecule  of 
ammonia.  It  may  also  be  applied  to  those  poly-acid  bases 
whose  acid  equivalence  is  known.  The  molecular  weights  of 
basic  substances  may  be  ascertained  with'  equal  certainty  by 
determining  how  much  of  them  is  required  to  neutralize  known 
quantities  of  acids.  In  this  case,  also,  the  acid-equivalence  of 
(•lie  base,  i.e.  the  number  of  molecules  of  a  mono-basic  acid  it 
can  neutralize,  must  be  known. 


DKTKIIMIN  ATION    OF    M<  >LK<  '(  !LAK   WEIGHTS          157 


will  serve  to  illustrate  the  application  of  the  chemi- 
cal method  of  determining  molecular  weights  to  substances 
which  are  neither  acids  nor  bases.  An  analysis  shows  that  in 
it  C  :  II  ::  1  :  1.  The  simplest  formula  which  the  compound  can 
have  is,  therefore,  CH.  It  is  found,  however,  on  treating  ben- 
zene with  chlorine  that  the  hydrogen  in  it  is  replaced  in  several 
different  stages,  and  that  in  the  compound  containing  the  least 
chlorine  only  one  sixth  of  the  hydrogen  has  been  replaced. 
The  molecule  of  benzene  cannot,  therefore,  contain  less  than 
six  atoms  of  that  element.  Moreover,  since  the  quantities  of 
hydrogen  replaced  by  chlorine,  or  any  other  element  or  group 
of  elements,  always  vary  by  one  or  more  sixths  of  the  whole  and 
never  by  any  smaller  fraction,  it  is  extremely  improbable  that 
the  molecule  contains  more  than  six  atoms  of  hydrogen.  Hence 
the  formula  C6H0  must  be  assigned  to  benzene. 

PHYSICAL  METHODS 

The  so-called  "  physical  methods  "  of  determining  molecular 
weights  are  all  based  on  what  are  called  by  Ostwald  the  colr 
ligative  properties  of  matter.  Under  certain  conditions  equal 
numbers  of  molecules  produce  equal  effects,  regardless  of  the 
chemical  constitution  and  character  of  the  molecules.  Thus 
the  volumes  of  gases  under  equal  conditions  of  temperature 
and  pressure  are  proportional  to  the  number  of  gaseous  mole- 
cules, and  wholly  independent  of  the  composition  of  the  mol- 
ecules. The  pressures  exerted  by  equal  volumes  of  gases  at 
the  same  temperature  are  likewise  proportional  to  the  number 
of  molecules  which  they  contain.  The  occupation,  under  the 
same  conditions  of  temperature  and  pressure,  of  equal  spaces 
by  equal  numbers  of  gaseous  molecules,  without  regard  to  their 
weight  or  chemical  character,  is  a  "  colligative  "  property  of 
gases.  If  equal  volumes  of  different  gases,  when  measured 
under  like  conditions,  contain  the  same  number  of  molecules, 
the  weights  of  such  equal  volumes  are  related  to  each  other  as 


158  QUANTITATIVE  EXERCISES 

are  the  molecular  weights  of  the  gases,  and  the  determination 
of  the  density  of  a  gas  is  equivalent  to  a  determination  of 
its  molecular  weight. 

Other  colligative  properties  of  matter  on  which  may  be  founded 
methods  of  determining  molecular  weights  are  observed  in  cer- 
tain effects  which  are  produced  by  substances  in  solution.  The 
effects  known  as  osmotic  pressure,  the  lowering  of  the  vapor 
tension  of  the  solvent,  and  the  depression  of  its  freezing  point 
are  all  proportional  not  to  the  weight  of  the  different  substances 
dissolved  but  to  the  number  of  the  molecules  contained  in  a 
unit  volume  of  their  solution.  Under  the  same  conditions  equal 
numbers  of  molecules  in  a  unit  volume  of  a  solution  produce 
equal  osmotic  pressure,  an  equal  lowering  of  the  vapor  tension 
of  the  solvent,  and  an  equal  depression  of  its  freezing  point. 
Hence  we  may  determine  the  relative  molecular  weights  of  dif- 
ferent substances  by  finding  what  quantities  of  them  will  pro- 
duce these  effects  to  the  same  degree;  since  these  quantities 
are  related  to  each  other  as  are  the  molecular  weights  of  the 
substances  themselves. 


I.   Dumas'  Method 

At  the  present  time  the  method  of  Dumas  is  rarely  resorted 
to  by  chemists ;  it  will  therefore  not  be  described  in  great  detail. 

By  this  method,  which  is  applicable  to  easily  volatilized  liquids 
and  solids,  the  molecular  weight  of  a  substance  is  deduced  from 
the  weight  of  a  known  volume  of  its  vapor.  The  principle  on 
which  it  is  based  is  precisely  the  same  as  that  which  is  employed 
in  determining  the  densities  and  the  molecular  weights  of  gases 
by  weighing.  It  is,  in  fact,  a  method  for  the  determination  of  the 
densities  of  the  vapors  of  substances  at  temperatures  somewhat 
above  their  boiling  points. 

A  glass  balloon,  Fig.  30,  having  a  capacity  of  from  100  cc.  to 
250  cc.,  is  carefully  cleansed,  and  dried  internally  by  attaching 
to  it  a  calcium  chloride  tube  through  which  air  is  alternately 


DETERMINATION  OF    MOLECULAR   WEIGHTS          159 

pumped  out  and  readmitted.  The  balloon  is  weighed,  with  a 
closed  vessel  of  the  same  form  and  size  as  counterpoise,  and 
the  temperature  of  the  air  within  the  balance  case  and  the 
height  of  the  barometer  are  noted.  The  balloon  is  then  warmed 
and  its  outlet  is  immersed  in  the  liquid  whose  density  is  to  be 
determined.  On  cooling,  the  liquid  ascends  into  the 
balloon.  After  the  introduction  of  several  grams 
of  the  material,  the  balloon  is  placed  in  a  bath 
which  can  be  heated  to  a  temperature  considerably 
higher  than  the  boiling  point  of  the  substance. 
Only  the  outlet  is  allowed  to  project  above  the 
liquid  in  the  bath.  The  bath  is  heated  to  the  re- 
quired temperature  and  constantly  stirred.  When 
nearly  all  of  the  liquid  has  distilled  out  of  the  balloon,  the  tem- 
perature of  the  bath  is  kept  as  constant  as  possible  until  no  more 
vapor  issues  from  the  apparatus.  The  outlet  is  then  closed  with 
a  blowpipe,  and  a  record  is  made  of  the  temperature  of  the  bath 
and  of  the  height  of  the  barometer.  The  balloon  is  removed 
from  the  bath,  cleansed,  and  weighed,  and  the  temperature  within 
the  balance  case  at  the  time  of  weighing  and  the  height  of  the 
barometer  are  again  recorded.  To  determine  the  capacity  of 
the  balloon,  a  file  mark  is  made  on  the  small  tube  near  the  end ; 
the  end  of  the  tube  is  then  immersed  in  mercury  or  recently 
boiled  water,  and  the  tip  broken  off.  The  liquid  rises  in  the 
balloon,  and  should  fill  it  completely,  excepting,  of  course,  the 
space  occupied  by  the  liquefied  or  solidified  substance  under 
investigation.  As  a  rule,  however,  a  bubble  of  gas  remains  in 
the  top  of  the  bulb,  showing  that  the  air  was  not  all  expelled 
by  the  vapors  of  the  substance.  The  space  occupied  by  the  sub- 
stance is  so  small  that  it  may  safely  be  neglected,  but  the 
volume  of  the  residual  air  must  usually  be  determined.  The  pro- 
cedure by  which  the  capacity  of  the  apparatus  and  the  volume 
of  the  air  bubble  are  both  ascertained  is  as  follows  :  The  balloon 
is  submerged  in  the  liquid  until  the  air  within  is  under  atmos- 
pheric pressure,  i.e.  until  the  liquid  within  and  that  without  are 


160  QUANTITATIVE  EXERCISES 

on  the  same  level.  The  outlet  is  then  closed  with  the  finger  and 
the  bulb  removed  from  the  bath.  The  bulb  is  inverted  and  the 
air  allowed  to  escape.  If  mercury  has  been  used,  the  quantity 
of  that  metal  required  to  complete  the  filling  of  the  vessel  is 
determined  either  by  weighing  or  by  measuring.  The  volume 
of  the  mercury  thus  introduced  is  equal  to  that  of  the  air  bubble. 
Finally,  the  capacity  of  the  vessel  is  found  by  measuring  or 
weighing  the  mercury  which  now  completely  fills  it.  If,  on  the 
other  hand,  water  has  been  used,  the  vessel,  after  the  escape  of 
the  air,  is  dried  externally  and  weighed.  It  is  then  completely 
filled  and  again  weighed.  The  difference  between  the  two 
weights  is  the  weight  of  the  water  whose  volume  is  equal  to 
that  of  the  air  bubble.  The  capacity  of  the  vessel  is  calculated 
from  the  weight  of  the  water  which  fills  it.  Having  found  the 
capacity  of  the  balloon  at  the  temperature  of  the  last  experi- 
ment, its  capacity  at  the  temperature  of  the  bath  when  it  was 
closed  with  the  blowpipe  is  calculated  in  the  usual  manner. 
But  there  must  be  deducted  from  this  the  volume,  at  the  same 
temperature,  of  the  air  which  was  not  expelled  by  the  vapors 
of  the  substance.  The  difference  is  the  volume  of  the  vapor  at 
the  temperature  of  the  bath.  As  both  the  temperature  of  the 
bath  and  the  height  of  the  barometer  were  recorded  at  the  time 
of  closing  the  balloon,  the  theoretical  volume  of  the  vapor  under 
standard  conditions  can  be  calculated. 

The  weight  of  the  vapor  is  found  by  deducting  from  that 
of  the  closed  apparatus  both  the  weight  of  the  bulb  and  that 
of  the  air  which  remained  in  it  at  the  time  of  closing. 

Having  found  the  weight  of  the  vapor  and  its  theoretical  vol- 
ume under  standard  conditions  of  temperature  and  pressure,  the 
former  is  divided  by  the  weight  of  an  equal  volume  of  hydrogen 
under  the  same  conditions.  The  quotient  is  the  density  of  the 
vapor  as  compared  with  that  of  hydrogen.  To  find  the  molecular 
weight  of  the  substance,  its  vapor  density  must  be  doubled. 

The  weight  of  the  balloon  enters  twice  into  the  calculation  — 
first  in  finding  its  capacity,  and  secondly  in  finding  the  weight 


DETERMINATION  OF  MOLECULAR  WEIGHTS         161 

of  the  vapor.  But  when  the  balloon  was  weighed  in  the  first 
instance  it  was  filled  with  air.  To  find  its  true  weight,  there- 
fore, we  must  deduct  that  of  the  air  which  was  weighed  with  it. 

For  the  determination  of  the  molecular  weights  of  substances 
whose  boiling  points  are  too  high  to  permit  the  use  of  glass, 
porcelain  balloons  have  been  employed.  To  close  these,  the 
flame  of  the  oxy-hydrogen  blowpipe  is  required. 

The  materials  used  in  the  bath  must  be  such  as  to  permit  the 
raising  of  the  temperatures  of  the  vapors  several  degrees  above 
the  boiling  points  of  the  substances,  since  at  temperatures  near 
their  boiling  points  the  vapors  of  substances  do  not  sufficiently 
obey  the  laws  of  Boyle  and  of  Gay-Lussac. 

II.  The  Method  of  Gay-Lussac 
(Hofmann's  Modification) 

The  method  of  Gay-Lussac  is  the  converse  of  Dumas',  since 
by  it  the  molecular  weight  is  deduced  not  from  the  weight 
of  a  known  volume  of  the  vapor  but  from  the  volume  of  the 
vapor  of  a  known  weight  of  the  substance. 

The  procedure  of  the  author  of  the  method  was  as  follows  : 
A  gas-measuring  tube  about  400  mm.  in  length  was  filled  with 
mercury  and  inverted  in  an  iron  bath  half  full  of  the  same 
metal.  A  thin  glass  bulb  completely  filled  with  a  known 
weight  of  the  substance  was  introduced  by  placing  it  under  the 
open  end  of  the  tube.  A  glass  cylinder,  open  at  both  ends, 
also  wider  and  somewhat  longer  than  the  measuring  tube,  was 
placed  in  the  bath  and  the  space  between  the  eudiometer  and 
the  cylinder  was  filled  with  water,  oil,  paraffin,  glycerin,  or 
some  other  liquid  which  could  be  heated  to  a  sufficiently  high 
temperature.  The  bath  was  then  heated  to  the  desired  temper- 
ature and  kept  as  nearly  constant  as  possible  until  the  volume 
of  the  vapor  no  longer  increased.  Finally  there  were  recorded 
(1)  the  volume  of  the  vapor,  (2)  the  length  of  the  mercury  col- 
umn within  the  measuring  tube,  (3)  the  temperature  of  the  bath, 


162  QUANTITATIVE  EXERCISES 

and  (4)  the  height  and  temperature  of  the  barometer.  From 
these  data  the  theoretical  volume  of  the  vapor  under  standard 
conditions  could  be  calculated ;  and  the  weight  of  the  substance 
divided  by  the  weight  of  an  equal  volume  of  hydrogen  gave  the 
theoretical  density  of  the  vapor  under  the  same  conditions. 

The  method  in  its  original  form  is  no  longer  in  use.  The 
principal  objections  to  it  are  (1)  the  mercury  bath,  the  vapors 
of  which  are  poisonous ;  (2)  the  difficulty  of  obtaining  liquids  to 
place  between  the  measuring  tube  and  the  cylinder,  which  will 
remain  transparent  at  high  temperatures ;  and  (3)  the  shortness 
of  the  measuring  tube.  The  method  could  be  used  only  for  the 
determination  of  the  molecular  weights  of  substances  having 

comparatively  low  boiling  points.     Most  of  these  diffi- 
"~~Jl      culties  were  overcome  by  a  modification  of  the  method 

which  was  proposed  by  Hofmann. 


Hofmann's  Method 

In  the  apparatus  of  Hofmann,  Fig.  31,  the  measuring 
tube  has  a  length  of  one  meter.  The  advantage  of  the 
greater  length  is  in  the  effect  of  diminished  pressure  on 
the  boiling  points  of  the  substances  whose  molecular 
weights  are  sought.  By  enlarging  the  closed  end  of  the 
tube  and  thus  increasing  the  vacant  space  into  which  the 
substance  evaporates,  or  by  using  very  small  quantities 
FIG  31  °^  material»  it  is  practicable  to  determine  the  molecular 
weights  of  substances  at  temperatures  several  degrees 
below  their  boiling  points  under  ordinary  atmospheric  pressure. 
A  glass  jacket  40  mm.  wide  and  90  cm.  long  is  placed  over 
the  measuring  tube  in  the  position  shown  in  the  figure,  and 
connected  at  the  top  with  a  vessel  containing  a  liquid  of  suit- 
able boiling  point.  The  vapor  of  the  boiling  liquid  enters  the 
jacket  and  passes  out  through  the  small  tube  at  its  lower  end, 
heating  the  tube  to  the  boiling  point  of  the  liquid.  The  sub- 
stance is  inclosed  in  a  minute  flask  (with  a  ground-glass  stopper), 


DETERMINATION  OF  MOLECULAR  WEIGHTS         163 

which  is  just  large  enough  to  contain  the  quantity  of  material 
required  for  the  determination. 

The  experiment  is  made  in  the  following  manner :  The  meas- 
uring tube  is  filled  with  mercury,  inverted,  and  its  open  end 
immersed  in  a  dish  of  the  same  metal.  The  height  and  temper- 
ature of  the  barometer  and  of  the  mercury  column  in  the  tube 
are  recorded.  The  little  flask  containing  the  weighed  quantity 
of  the  substance,  which  must  completely  fill  it,  is  brought 
under  the  open  end  of  the  tube  and  allowed  to  ascend  into  the 
vacuum  above.  As  it  reaches  the  top  of  the  mercury  column, 
the  stopper  usually  flies  out  of  the  flask.  It  is,  however,  imma- 
terial whether  it  does  so  or  not,  since,  unless  the  flask  has  been 
too  tightly  closed,  it  is  soon  expelled  by  the  expanding  material. 
The  jacket  is  connected  at  the  top  with  a  flask  or  other  vessel 
containing  the  liquid  whose  vapor  is  to  heat  the  substance,  and 
the  exit  tube  at  the  bottom  is  attached  to  a  condenser  if  nec- 
essary. The  liquid  employed  depends  on  the  temperature 
required.  The  liquids  most  frequently  used  are  water,  aniline 
(183°),  naphthalene  (217°),  ethyl-benzoate  (213°),  and  amyl- 
benzoate  (261°).  The  liquid  in  the  vessel  connected  with  the 
jacket  is  boiled  until  the  volume  of  the  vapor  in  the  measuring 
tube  no  longer  increases.  There  are  then  recorded  (1)  the  vol- 
ume of  the  vapor,  (2)  the  length  of  the  mercury  column  above 
the  lower  end  of  the  jacket,  (3)  the  length  of  the  mercury 
column  below  the  jacket,  i.e.  that  portion  of  the  column  which 
is  not  heated  by  the  vapor  of  the  boiling  liquid,  (4)  the  tem- 
perature, as  nearly  as  it  can  be  determined,  of  the  mercury  in 
the  tube  at  a  point  halfway  between  the  lower  end  of  the 
jacket  and  the  surface  of  the  metal  in  the  dish,  and  (5)  the 
height  and  temperature  of  the  barometer. 

In  finding  the  pressure  under  which  the  vapor  was  measured 
(by  subtracting  the  corrected  height  of  the  mercury  column  in 
the  measuring  tube  from  the  corrected  height  of  the  barom- 
eter) the  two  portions  of  the  mercury  column,  having  different 
temperatures,  must  be  separately  reduced  to  their  length  at 


164  QUANTITATIVE  EXERCISES 

0°.  It  will  be  seen  that  the  correction  of  the  height  of  the 
mercury  column  is  at  best  only  approximate,  owing  to  the  lack 
of  uniformity  in  the  temperature  of  its  different  parts.  Several 
plans  for  overcoming  this  difficulty  have  been  proposed.  Of 
these,  the  most  obviously  practicable  one  is  to  make  the  mer- 
cury cistern  narrower  and  so  to  lengthen  the  jacket  that  it 
will  inclose  both  the  eudiometer  and  the  cistern.  The  correc- 
tion for  the  tension  of  the  mercury  vapor  is  made  by  subtract- 
ing from  the  corrected  pressure  the  tension  of  the  metal  for  the 
given  temperature  as  found  in  the  tables.  The  measuring  tube 
should  be  calibrated,  and  if  any  considerable  degree  of  accuracy 
is  required,  a  correction  for  the  expansion  of  glass  must  be 
applied. 

The  use  of  the  Gay-Lussac  method  and  of  the  Hofmann  modi- 
fication of  it  is  necessarily  limited  to  substances  whose  boiling 
points  are  considerably  below  that  of  mercury.  At  temperatures 
above  300°  it  cannot  be  employed  with  advantage  owing  to  the 
great  tension  of  the  mercury  vapor. 

Determination  of  the  Volume  of  a  Vapor  by  Means  of  the 
Pressure  which  it  Exerts 

The  volume  which  a  gas  would  have  under  standard  condi- 
tions of  temperature  and  pressure  may  be  deduced  from  the 
pressure  which  it  is  found  to  exert  when  confined,  at  any  given 
temperature,  in  a  space  of  known  volume.  Hence  the  molecular 
weight  of  a  volatile  substance  may  be  ascertained  by  determining 
the  pressure  exerted  by  a  known  weight  of  its  vapor  in  a  vessel 
of  known  capacity.  Modifications  of  the  Gay-Lussac  method, 
which  are  based  upon  this  principle,  have  been  proposed  by  Bell 
and  Teed  (Journal  of  the  Chemical  Society,  1880,  1,  576),  and 
by  Malfatti  and  Schoop  (Zeitschrift  fur  physikalische  Chemie, 
1887,  159). 


.    DETERMINATION  OF  MOLECULAR  WEIGHTS         165 

Determination  of  the  Volume  of  a  Vapor  by  measuring  the 
Liquid  or  the  Gas  which  it  Displaces 

Methods  of  this  kind  are  identical  in  principle  with  that 
of  Gay-Lussac,  since  they  differ  from  it  only  in  the  manner  of 
determining  the  volume  of  the  known  weight  of  vapor.  This 
is  ascertained  indirectly  by  measuring  the  volume  of  a  liquid 
(usually  mercury)  or  of  a  gas  (air,  hydrogen,  or  nitrogen)  which 
the  vapor  displaces.  The  best  known  and  most  useful  of  these 
methods  is  that  of  Victor  Meyer. 


EXERCISE  XIII 

DETERMINATION  OF  THE  MOLECULAR  WEIGHT  OF 
CHLOROFORM  BY  THE  METHOD  OF  MEYER 

The  simplest  form  of  the  Meyer  apparatus  is  shown  in  Fig.  32. 
It  consists  of  three  parts :  (1)  a  glass  tube  in  which  a  weighed 
quantity  of  the  substance  is  converted  into  a  vapor ; 
(2)  a  glass,  or  better,  in  many  cases,  a  metallic, 
mantle  or  bath,  in  the  bottom  of  which  is  placed  the 
liquid  to  be  heated  for  the  purpose  of  producing  the 
required  temperature ;  and  (3)  a  graduated  tube  in 
which  is  collected  the  air  or  other  gas  displaced  by 
the  vapor  of  the  substance.  The  total  length  of  part 
1  is  about  800  mm.  Its  lower  and  larger  end  b  has 
a  capacity  of  about  100  cc.  The  narrower  portion, 
between  b  and  the  cuplike  enlargement  at  the  top, 
has  an  internal  diameter  of  about  6  mm.  The  side 
tube  a,  through  which  the  displaced  gas  passes  into 
the  measuring  tube,  is  joined  to  the  main  tube  at  a 
point  about  100  mm.  from  the  upper  end  of  the  latter. 
Its  internal  diameter  should  not  much  exceed  1  mm. 
The  mantle  has  a  diameter  of  40  mm.  and  a  length  of 
520  mm.,  exclusive  of  the  bulblike  enlargement  at  \HJ 
the  lower  end,  the  capacity  of  which  is  about  80  cc.  FIG.  32 


166  QUANTITATIVE  EXERCISES 

Thoroughly  cleanse  the  apparatus  by  washing  it  with  alcohol, 
and  then  with  ether  which  leaves  110  residue  011  evaporation. 
Attach  a  calcium  chloride  tube  to  a,  insert  a  stopper  through 
which  is  passed  a  long  glass  tube  reaching  to  the  bottom  of  6, 
and  aspirate  air  through  the  apparatus  until  it  is  dry.  Place  a 
quantity  of  ignited  asbestus  in  the  bottom  of  b.  Fill  the  bulb  c 
nearly  full  of  distilled  water,  and  clamp  the  mantle  in  an  iron 
stand  at  such  a  height  that  when  the  stand  is  upon  the  floor, 
the  tube  a  will  be  a  little  above  the  top  of  the  work  table. 
Arrange  all  parts  of  the  apparatus  as  indicated  in  the  figure. 
Select  a  tightly  fitting,  singly  perforated  rubber  stopper  for  d. 
Pass  through  it,  from  the  upper  end,  a  short  glass  tube  provided 
with  a  ground  stopcock.  The  tube  need  not  have  an  external 
diameter  of  more  than  5  mm.  The  channel  through  the  stopcock 
should,  however,  be  relatively  large.  Blow  a  bulb  with 
a  capacity  of  0.3  or  0.4  cc.  on  the  end  of  a  thin  small 
glass  tube,  Fig.  33.  Weigh  the  bulb,  immerse  the  open 
end  of  the  stem  in  pure  chloroform,  and,  by  alternately 
warming  and  cooling  the  bulb,  introduce  from  0.2  to 
F  33  0.3  cc.  Dry  the  wet  end  of  the  stem  with  filter  paper 
and  close  it  in  the  flame.  Weigh  again.  Pass  the  stem 
of  the  bulb  through  the  stopcock,  as  shown  in  Fig.  33,  and  fix 
it  in  position  by  turning  the  latter  until  it  presses  lightly  against 
the  stem.  Insert  a  stopper  in  d.  Boil  the  water  in  c  vigorously. 
Immerse  the  outlet  of  a  in  the  dish  of  water  e.  When  no  more 
air  issues  from  the  apparatus,  fill  the  gas-measuring  tube  with 
water  and  invert  it  over  the  outlet  of  a.  Replace  the  stopper 
in  d  by  the  arrangement  represented  in  Fig.  33,  and,  after  about 
one  minute,  close  the  stopcock.  The  stem  will  be  cut  and  the 
bulb  containing  the  chloroform  will  fall  upon  the  asbestus  in 
the  bottom  of  b. 

The  chloroform  is  converted  into  vapor,  and  a  volume  of  air 
equal  to  that  of  the  vapor  is  transferred  from  the  upper  part  of 
the  apparatus  to  the  measuring  tube.  The  air  in  passing  through 
the  water  is  cooled,  and  its  volume  when  measured  is  therefore 


OF  MOLECULAR  WEIGHTS         167 

smaller  than  that  of  the  vapor  of  the  chloroform.  This,  how- 
ever, is  not  a  source  of  error,  since,  if  the  air  transferred  and 
the  chloroform  vapor  were  of  the  same  temperature,  the  latter 
would  theoretically  have  suffered  the  same  contraction  in  con- 
sequence of  an  equal  lowering  of  temperature.  The  condition 
that  the  air  forced  out  of  the  apparatus  shall  have  the  same 
temperature  as  the  vapor  whose  density  is  to  be  determined  is 
an  essential  one,  for  if  its  temperature  is  lower,  the  contraction 
in  passing  through  the  water  will  be  too  small,  and  the  volume 
of  the  air  collected  in  the  measuring  tube  will  therefore  be  too 
large.  It  cannot,  however,  be  fully  realized,  since  the  air  in  the 
outlet  tube,  which  is  the  first  to  be  forced  into  the  eudiometer, 
has  a  temperature  very  near  that  of  the  outside  atmosphere.  It 
is  for  this  reason  that  the  bore  of  a  is  made  as  small  as  is 
practicable. 

The  heating  is  to  be  continued  until,  for  a  space  of  two  min- 
utes, no  more  air  is  expelled.  The  air  is  then  measured  and  its 
volume  under  standard  conditions  is  found  in  the  usual  way. 
The  weight  of  the  chloroform  divided  by  the  weight  of  an  equal 
volume  of  hydrogen  is  the  theoretical  density  of  the  chloroform, 
and  its  density  is  one-half  its  molecular  weight. 


The  original  apparatus  of  Meyer  was  closed  with  a  simple 
rubber  stopper,  which  was  removed  at  the  time  of  introducing 
the  substance  and  then  quickly  replaced.  The  compression  pro- 
duced by  the  insertion  of  the  stopper  caused  the  expulsion  of  a 
few  bubbles  of  air  which  were  allowed  to  escape  before  placing 
the  eudiometer  in  position  over  the  outlet.  Under  these  con- 
ditions the  success  of  the  determination  depended  very  much 
upon  the  good  judgment  and  the  alertness  of  the  experimenter. 
Hence  a  number  of  modifications  of  the  apparatus  were  soon 
proposed,  which  obviate  the  necessity  of  opening  it  at  the  time 
of  dropping  the  substance  into  the  heated  chamber  below.  The 
accompanying  Fig.  34,  A,  B,  and  (7,  represents  some  of  the 


168 


QUANTITATIVE  EXERCISES 


modifications  which  are  now  in  common  use.  In  -4,  the  tube 
containing  the  substance  is  released  by  turning  the  bent  wire 
which  passes  through  the  stopper ;  in  B,  by  turning  the  stop- 
cock; and  in  C,  by  drawing  to  the  left  the  wire  in  the  hori- 
zontal tube.  A  distinct  disadvantage  in  all  of  these  devices  is 
the  necessity  of  introducing  the  substances  in  open  tubes,  and  of 
leaving  them  for  some  time  exposed  to  a  more  or  less  elevated 
temperature.  If  tubes  with  ground-glass  stoppers  are  substi- 
tuted for  the  open  ones,  the  determinations  are  often  lost  in 
consequence  of  their  failure  to  release  the  stoppers  even  when 
heated  above  the  boiling  points  of  the  substances. 

These  difficulties  are  obviated  in  the  case  of  liquids  by  using 
the  arrangement  prescribed  in  the  preceding  exercise.    If  the 

fe 


A 


V 


substance  is  a  solid,  a  narrow  tube  is  closed  at  one  end  and 
then  drawn  out  at  a  point  near  the  closed  end,  leaving  a  pas- 
sage just  large  enough  for  the  introduction  of  the  material. 
The  tube  is  weighed^  the  substance  introduced,  and  its  weight 
ascertained.  The  tube  is  then  fused  off  at  the  proper  distance 
from  the  substance,  leaving  a  stem  which  is  passed  through  the 
stopcock  in  the  manner  already  described. 

When  a  substance  is  liable  to  be  attacked  by  oxygen  at  the 
temperature  required  for  the  determination  of  its  molecular 
weight,  the  air  in  the  apparatus  is  displaced  by  some  other  gas, 
such  as  nitrogen  or  hydrogen.  And  when  the  temperatures  are 


DETERMINATION  OF  MOLECULAR  WEIGHTS          169 

too  high  to  permit  the  use  of  glass  vessels,  an  apparatus  of 
platinum  or  of  porcelain  is  employed.  The  following  list  con- 
tains the  names  and  boiling  points  of  some  of  the  substances 
which  are  used  in  the  bath  of  the  Victor  Meyer  apparatus. 
Water  (100°),  amyl-alcohol  (130°),  xylene  (137°  to  143°),  aniline 
(182.5°),  ethyl-benzoate  (213°),  amyl-benzoate  (261°),  diphenyl 
amine  (310°),  mercury  (357.25°),  sulphur  (448.4°),  phosphorus 
pentasulphide  (518°),  stannous  chloride  (606°).  For  the  deter- 
mination of  molecular  weights  at  still  higher  temperatures,  a 
gas  or  a  coal  furnace  is  employed. 

THE  FREEZING-POINT  METHOD 

When  a  substance  is  dissolved  in  a  liquid  the  freezing  point 
of  the  latter  is  lowered,  and  the  observed  depression  is,  in  gen- 
eral, very  nearly  proportional  to  the  concentration  of  the  solu- 
tion. In  other  words,  if  one  gram  of  sugar  is  dissolved  in  100 
grams  of  water,  the  latter  will  no  longer  freeze  at  0°,  the  freez- 
ing point  of  pure  water,  but  at  some  lower  temperature ;  if  two 
grams  of  sugar  are  dissolved  in  the  same  quantity  of  water,  the 
observed  depression  of  the  freezing  point  will  be  double  that  of 
the  solution  containing  one  gram,  etc. 

The  depression  of  the  freezing  point  which  is  observed  in  a 
one-per-cent  solution  of  a  substance  in  any  given  liquid,  i.e. 
when  one  gram  of  the  substance  is  dissolved  in  100  grams  of 
the  liquid,  is  called  the  specific  depression  of  the  freezing  point 
of  the  liquid  for  the  particular  substance. 

If  different  substances,  in  quantities  proportional  to  their 
respective  molecular  weights,  are  dissolved  in  equal  weights  of 
any  given  liquid,  the  different  solutions  will  as  a  rule  be  found 
to  have  the  same  freezing  point.  From  this  fact  is  drawn  the 
conclusion  that  the  extent  of  the  depression  in  the  case  of  any 
given  liquid  is  determined  solely  by  the  numerical  ratio  of  the 
molecules  of  the  dissolved  substance  and  of  the  solvent.  In 
other  words,  equal  numbers  of  molecules,  when  dissolved  in 


170  QUANTITATIVE  EXERCISES 

equal  quantities  of  any  given  liquid,  produce  an  equal  lowering 
of  the  freezing  point  whether  the  molecules  are  of  the  same 
kind  or  of  different  kinds. 

The  depression  of  the  freezing  point  which  results  from  dis- 
solving in  100  grams  of  a  liquid  a  number  of  grams  equal  to  its 
molecular  weight  is  called  the  molecular  depression  of  the  freez- 
ing point  of  the  liquid  for  that  substance.  The  molecular 
depression  is  equal  to  the  product  of  the  specific  depression  and 
the  molecular  weight  of  the  substance.  For  any  given  liquid  the 
molecular  depressions  for  different  substances  have  a  nearly  con- 
stant value.  This  constant  may  be  determined  experimentally 
by  dissolving  in  a  known  weight  of  a  liquid  a  weighed  quantity 
of  some  substance  of  known  molecular  weight,  and  ascertaining 
the  depression  of  the  freezing  point ;  or  it  may  be  calculated,  when 
the  requisite  data  are  at  hand,  by  the  equation  of  Van't  Hoff : 

T2 
C=  0.02 — ,  in  which 

C  is  the  constant  to  be  found, 

T  the  absolute  freezing  point  of  the  liquid,  and 

W  its  latent  heat  of  fusion. 

The  following  table  gives  the  molecular  depression  of  the 
freezing  point  of  several  liquids  as  found  by  experiment  and  by 
calculation. 

BY  EXPERIMENT  BY  CALCULATION 

Water 18.8 18.7 

Acetic  acid 39.0 38.8 

Formic  acid 28.0 28.4 

Benzene 50.0 53.0 

Nitrobenzene 70.0 69.5 

Phenol 74.0 77.0 

Naphthalene    ., 69.0 69.4 

If  the  molecular  depression  of  the  freezing  point  of  any 
solvent  is  divided  by  the  molecular  weight  of  the  same,  the 
quotient  is  the  depression  which  one  molecule  of  any  substance, 
behaving  normally  in  solution,  will  produce  in  100  molecules  of 


DETERMINATION   OF  MOLECULAR   WKKJIITS          171 

the  solvent.  This  value  is,  of  course,  constant  for  any  given 
solvent,  but,  contrary  to  the  conviction  of  Raoult,  it  varies  with 
different  solvents. 

There  are  many  exceptions  to  the  rule  that  equi-molecular 
quantities  of  different  substances  produce,  in  a  fixed  quantity 
of  any  given  solvent,  an  equal  lowering  of  the  freezing  point. 
For  example,  when  water  is  used  as  a  solvent,  molecular-equiv- 
alent quantities  of  the  class  of  bodies  known  as  electrolytes  — 
acids,  bases,  and  salts  —  produce  a  much  greater  depression  of 
the  freezing  point  than  equivalent  quantities  of  indifferent  sub- 
stances, such  as  the  sugars  and  urea.  Hydrochloric,  hydrobromic, 
hydriodic,  and  nitric  acids,  also  the  hydroxides  and  the  halogen 
salts  of  the  metals  of  the  alkalies,  produce  in  dilute  aqueous 
solutions  nearly  twice  the  normal  depression  of  the  freezing 
point.  This  conduct  on  the  part  of  electrolytes  is  not  regarded 
as  invalidating  the  law  that  equal  numbers  of  molecules  pro- 
duce equal  effects.  It  is  supposed,  rather,  to  indicate  that,  in 
aqueous  solutions,  the  molecules  of  the  bodies  in  question  suffer 
a  certain  kind  of  dissociation.  In  benzene,  nitrobenzene,  and 
ethylene  bromide,  on  the  other  hand,  the  alcohols  and  the 
organic  acids  produce  only  about  one-half  the  normal  depres- 
sion of  the  freezing  point.  This  conduct  is  supposed  to  be  due 
to  the  existence  in  such  solutions  of  double  molecules,  e.g. 
(C2H4O2)2,  instead  of  C2H4O2,  since  the  same  substances  in 
other  solvents  produce  normal  depressions. 


To  determine  the  molecular  weight  of  any  substance  by  the 
freezing-point  method,  a  small  weighed  quantity  of  it  (g}  is  dis- 
solved in  a  weighed  quantity  of  some  solvent  (G)  in  which  it  is 
supposed  to  behave  normally,  and  whose  molecular  depression 
(c)  is  known.  The  freezing  point  of  the  solvent  is  determined 
before  (t)  and  after  (£,)  the  introduction  of  the  substance.  The 
depression  (t  —  1 1)  is  represented  by  d.  Having  found  the  depres- 
sion d,  which  g  grams  of  the  substance  produce  in  G  grams  of 


172  QUANTITATIVE  EXERCISES 

the  solvent,  it  is  necessary  to  find  how  much  of  it  would  produce 
an  equal  depression  in  100  grams  of  the  solvent,  since  the  con- 
stant  c  is  calculated  for  that  weight  of  the  liquid.  This  is  done 
by  means  of  the  proportion 


xslOO  ::*:(?.     x 

G 

If  100  —  grams  of  the  substance  produce  the  depression  d  in 
G 

100  grams  of  the  solvent,  the  quantity  (m)  which  will  produce 

the  molecular  depression  is  lOOc  —  ^-,  since 

Gd 


. 
G  Gd 

The  process  by  which  c  is  obtained  is  such  that  m  is  equal  to 

the  molecular  weight  of  the  substance.  The  formula  m  =  10  0  c  —- 

Gd 

is  general,  and  to  obtain  the  molecular  weight  of  any  substance 
it  is  necessary  only  to  substitute  in  it  the  proper  numerical 
values  of  (7,  #,  (7,  and  d. 

The  method  is  applicable  only  to  quite  dilute  solutions,  and 
appears,  in  general,  to  give  the  most  satisfactory  results  in 
those  which  are  about  one-tenth  normal. 

The  thermometer  which  is  used  both  in  the  freezing-point 
and  in  the  boiling-point  methods  of  determining  molecular 
weights  was  devised  by  Beckmann,  Fig.  35.  Its  characteristic 
feature  is  the  reservoir  at  the  top  to  which  a  portion  of  the 
mercury  in  the  bulb  may  be  transferred.  The  scale  is  divided 
into  hundredths  and  has  a  total  range  of  only  five  or  six 
degrees.  It  is  practicable,  by  transferring  portions  of  mercury 
from  the  bulb  below  to  the  reservoir  above,  or  from  the  reser- 
voir to  the  bulb,  to  adjust  the  instrument  for  use  at  widely 
different  temperatures.  Suppose,  for  instance,  that  it  is  required 
to  measure  changes  of  temperature  between  34°  and  38°,  and 
that  the  range  of  the  scale  is  nominally  6°.  The  bulb  is  placed 
in  water  having  a  temperature  of  about  40°.  If  the  mercury 


DETERMINATION  OF  MOLECULAR  WEIGHTS 


173 


FIG.  35 


rises  to  the  top  of  the  scale  or  slightly  above  it,  there  is  no 
need  of  readjustment,  since,  in  that  case,  the  upper  end  of  the 
mercury  column  will  be  within  the  scale  both  at  34°  and  38°. 
If  the  mercury  rises  and  flows  over  into  the  reservoir,  the 
column,  when  the  mercury  ceases  to  expand,  is  broken  off  at  its 
highest  point  by  striking  the  bulb  against  the  palm  of  the 
hand.  The  thermometer  will  then  register 
any  temperature  between  the  desired  limits. 
If,  on  the  other  hand,  the  mercury  on  being 
heated  to  40°  does  not  enter  the  scale,  or 
ascends  only  part  way  through  it,  a  portion 
of  the  metal  in  the  reservoir  must  be  trans- 
ferred to  the  bulb.  This  transference  is  accom- 
plished in  the  following  manner :  The  mercury 
above  is  forced  into  the  upper  end  of  the  res- 
ervoir by  striking  the  top  of  the  thermometer 
against  the  hand.  The  bulb  is  then  heated 
until  the  mercury  in  it  rises  and  joins  that  in 
the  reservoir.  Finally,  the  bulb  is  placed  in 
water  of  40°,  and  the  column  broken  in  the 
manner  previously  described. 

It  is  to  be  observed  that  the  divisions  upon 
the  scale  of  the  Beckmann  thermometer  have 
a  somewhat  variable  value  owing  to  the  fact 
that  the  quantity  of  mercury  in  the  bulb  varies 
according  to  the  temperatures  for  which  the 
thermometer  is  adjusted.  In  other  words,  the 
value  of  the  scale  as  a  measure  of  changes 
of  temperature  (of  depressions  and  elevations) 
depends  on  the  quantity  of  the  contracting  or  expanding  mer- 
cury in  the  bulb.  If  the  quantity  is  diminished,  as  it  must  be 
for  the  higher  temperatures,  the  value  of  the  scale  is  correspond- 
ingly increased ;  if,  on  the  other  hand,  a  portion  of  the  mercury 
is  transferred  from  the  reservoir  to  the  bulb  to  prepare  the 
instrument  for  use  at  lower  temperatures,  the  value  of  the  scale 


FIG.  36 


174  QUANTITATIVE  EXERCISES 

is  diminished.  Errors  from  this  source  may  require  attention 
when  a  thermometer  is  used  for  widely  separated  temperatures  ; 
as,  for  example,  when  the  same  instrument  is  employed  for  the 
determination  both  of  freezing  and  of  boiling  points. 

The  method  which  is  here  suggested  for  the  correction  of  such 
errors  is  as  follows :  The  value  of  the  scale,  when  the  mercury 
in  the  bulb  is  in  the  vicinity  of  0°,  is  determined  by  comparing 
the  instrument  with  a  standard  thermometer.  If  now  the  ther- 
mometer is  used  at  a  higher  temperature,  requiring  a  transfer  to 
the  reservoir  of  a  portion  of  the  mercury  in  the  bulb  at  0°,  the 
observed  depressions  or  elevations,  in  terms  of  the  value  which 
the  scale  has  at  0°,  are  to  be  multiplied  by  1  -f- 1°%  in  which 

t°  is  the  higher  temperature,  and 

7  the  apparent  expansion  of  mercury  in  glass,  i.e.  the  dif- 
ference between  the  cubical  expansion  of  mercury  and  of  glass. 
If,  on  the  other  hand,  the  instrument  is  used  at  a  lower  tem- 
perature, —  £°,  requiring  a  transfer  of  some  of  the  mercury  in 
the  reservoir  to  the  bulb,  the  observed  depressions  or  elevations 
are  to  be  multiplied  by  1  —  ^7. 

The  determination  of  the  value  of  the  scale,  when  the  mer- 
cury in  the  bulb  is  in  the  neighborhood  of  0°,  is  made  in  the 
following  manner:  The  mercury  above  is  forced  into  the  end 
of  the  reservoir  nearest  the  bulb,  and  the  mercury  in  the  bulb 
is  warmed  until  it  rises  and  forms  a  junction  with  that  in  the 
reservoir.  The  bulb  is  placed  in  water  having  a  temperature 
of  7°  or  8°,  and,  when  the  mercury  comes  to  a  standstill,  the 
thread  is  broken  off  at  its  highest  point.  The  bulb  is  now 
placed  in  ice  water.  The  top  of  the  thread  should  recede  nearly 
to  the  zero  mark  of  the  scale.  If  it  does  not,  or  if  it  descends 
below  that  point,  the  experiment  is  repeated,  using  warmer  or 
colder  water  for  the  first  immersion,  as  the  case  may  require. 

The  following  considerations  will  serve  to  explain  the  pro- 
posed method  of  correcting  the  Beckmann  thermometer.  Sup- 
pose an  instrument  of  this'kind  is  so  adjusted  that  the  end  of 
the  thread  is  within  the  scale  when  its  bulb  is  in  ice  water. 


DETERMINATION  OF  MOLECULAR  WEIGHTS          175 

Depressions  and  elevations  of  temperature  in  the  vicinity  of  0° 
can  now  be  measured  in  terms  of  the  scale.  Suppose,  however, 
it  is  desired  to  measure  changes  of  temperature  around  some 
higher  temperature  t°.  A  portion  of  the  mercury  in  the  bulb 
must  be  transferred  to  the  reservoir,  and  the  value  of  each  scale 
division  is  thereby  increased  in  the  same  proportion  as  the 
quantity  of  the  mercury  in  the  bulb  is  diminished.  If  we  let 
m  represent  the  quantity  of  mercury  in  the  bulb  when  the  instru- 
ment is  adjusted  for  0°,  then  the  quantity  in  the  bulb  when  it 
is  adjusted  for  the  higher  temperature  t°  will  be  m  —  mt°y. 
The  values  of  a  scale  division  are,  of  course,  inversely  propor- 
tional to  the  quantities  of  mercury  forced  into  the  stem  from  the 
bulb  in  consequence  of  a  given  rise  of  temperature,  and  these  are 
directly  proportional  to  the  quantities  in  the  bulb.  If  we  repre- 
sent by  unity  the  quantity  of  mercury  which  will  enter  the  stem 
in  consequence  of  a  given  rise  of  temperature  when  the  instru- 
ment is  adjusted  for  0°,  then  the  quantity  #,  which  will  enter 
when  it  is  adjusted  for  tc,  will  be  found  by  the  proportion 

m  :  m  —  mt°y : :  1  :  x. 
x  =  l-  t°y. 

That  is,  if  we  know  the  value  of  the  scale  when  the  temperature 
is  in  the  vicinity  of  0°,  we  can  find  its  value  in  the  vicinity  of 
any  higher  temperature  t°  by  multiplying  by  1  4-  t°y. 

In  the  same  way  it  may  be  shown  that,  knowing  its  value  in 
the  vicinity  of  0°,  we  can  find  its  value  at  any  lower  tempera- 
ture —  t°  by  multiplying  by  1  —  t°y. 

The  mercury  exhibits  a  strong  inclination  to  stick  in  the 
capillary  tube  of  the  Beckmann  thermometer.  This  tendency 
manifests  itself  more  strikingly  with  a  falling  than  with  a  rising 
temperature.  It  should  be  overcome,  as  far  as  possible,  when- 
ever a  reading  is  to  be  made,  by  tapping  the  thermometer  near 
the  top  with  some  hard  object.  A  device  for  this  purpose,  which 
has  been  used  with  advantage,  is  a  minute  vibrating  electrical 
hammer  attached  to  the  top  of  the  instrument. 


176 


QUANTITATIVE  EXERCISES 


Figures  36  and  37  represent  the  forms  of  the  Beckmann 
apparatus  which  are  in  common  use  for  the  determination  of 

molecular  weights  by  the  freez- 
ing-point method.  The  tube  A, 
Fig.  36,  which  contains  the  sol- 
vent whose  freezing  point  is  to 
be  determined  both  before  and 
after  the  introduction  of  the 
substance,  is  separated  from 
the  freezing  mixture  in  C by  the 
wider  tube  B.  The  substance 
to  be  investigated  is  intro- 
duced through  the  side  tube  Z>. 
The  stirrers  E  and  F,  the  for- 
mer of  which  is  best  made  of 
stout  platinum  wire,  are  em- 
ployed to  equalize  the  temper- 
ature of  the  liquid  in  A  and 
of  the  freezing  mixture  in  C. 
The  apparatus  represented  in 
Fig.  37  is  used  when  the  sol- 
vent, like  glacial  acetic  acid,  is 
hygroscopic.  The  only  essen- 
tial difference  between  it  and 
that  in  Fig.  36  is  the  arrangement  £,  through  which  dry  air  is 
passed  during  an  experiment.  The  tube  G  is 
of  such  internal  diameter  that  the  stirrer  which 
passes  through  it  can  be  worked  up  and  down 
without  friction.  To  its  side  is  attached  the  bulb 
tube  H,  in  which  a  small  quantity  of  concentrated 
sulphuric  acid  is  placed.  The  acid  is  prevented 
from  spattering  into  the  solvent  by  a  disk  which 
is  fastened  in  a  horizontal  position  to  the  wall  of 
the  larger  bulb.  The  air  is  dried  before  entering  H  by  passing  it 
over  pumice  stone  which  has  been  moistened  with  sulphuric  acid. 


FIG.  37 


FIG.  38 


DETERMINATION  OF  MOLECULAR  WEIGHTS         177 

Solid  substances  which  dissolve  readily  are  best  introduced 
in  the  form  of  compressed  tablets.  These  are  easily  prepared 
by  stamping  the  requisite  quantity  of  the  loose  material  in  a 
small  pastil  press  such  as  is  used  in  making  medicinal  tablets. 

Liquids  are  introduced  by  means  of  the  pipette  shown  in 
Fig.  38. 

EXERCISE   XIV 

DETERMINATION   OF   MOLECULAR   WEIGHTS   BY   THE 
FREEZING-POINT   METHOD 

1.  UREA 

Bring  into  the  tube  A  a  weighed  quantity  of  distilled  water 
which  will  somewhat  more  than  suffice  to  cover  the  bulb  of  the 
thermometer,  and  into  C  a  mixture  of  broken  ice,  coarse  salt,  and 
water.  Remove  A  from  B  and  immerse  it  for  a  time  in  the 
freezing  mixture.  Return  it,  after  drying  the  outside,  to  its 
place  in  B.  Let  the  water  in  it  cool  somewhat  below  the  freez- 
ing point,  and  then  stir  vigorously  until  ice  begins  to  form,  tak- 
ing care  not  to  let  the  stirrer  rub  against  the  bulb  of  the  ther- 
mometer. The  thermometer  will  rise  a  little  and  then  become 
stationary.  The  highest  temperature  is  the  true  freezing  point 
of  the  water.  Remove  the  tube  A  after  considerable  ice  has 
formed,  and  continue  to  stir  until  the  mercury  in  the  ther- 
mometer is  observed  to  rise.  Repeat  the  experiment  several 
times.  Remove  A  and  introduce  through  the  side  tube  in  com- 
pact form  a  quantity  of  pure  urea  which  is  calculated  to  give  a 
nearly  one-twentieth  molecular-normal  solution.  Stir  until  the 
urea  is  dissolved  and  the  ice  has  nearly,  but  not  quite,  all 
melted.  Return  A  to  its  place  in  B.  Let  the  solution  cool  for  a 
short  time  and  then  stir  until  ice  again  forms.  The  thermometer 
will  rise,  become  stationary  for  a  few  moments,  and  then  begin  to 
fall.  The  highest  temperature  reached  is  the  one  to  be  recorded. 
Remove  J,  stir  vigorously,  and  observe  the  thermometer  while 
the  ice  is  melting.  Repeat  the  experiment  several  times. 


178  QUANTITATIVE  EXERCISES 

Add  three  other  small  quantities  of  urea,  each  about  equal  to 
the  first,  and  determine  the  freezing  point  after  each  addition. 

Calculate  the  molecular  weight  of  urea  from  the  depression 
of  the  freezing  point,  which  was  observed  for  each  concentra- 
tion, using  18.9  as  the  molecular  depression  of  water.  Deduce 
also  from  the  observed  depressions  and  the  known  molecular 
weight  of  urea  the  molecular  depression  of  the  freezing  point 
of  water. 

If  stirring  the  under-cooled  solution  fails  to  induce  freezing, 
the  difficulty  may  be  overcome  by  introducing  a  minute  frag- 
ment of  ice.  The  effect  of  so  doing  upon  the  concentration  of 
the  solution  is,  of  course,  insignificant.  The  same  course  is  to 
be  followed  when  other  solvents  than  water  are  used.  If  no 
crystals  of  the  solvent  are  at  hand,  they  may  be  prepared  by 
freezing  a  little  of  the  pure  liquid  in  a  small  test  tube.  A  con- 
venient method  for  the  preparation  of  crystals  to  be  used  in 
starting  the  freezing  of  the  solvent  has  been  described  by  Beck- 
mann  (ZeitscJirift  fur  physikalisehe  Chemie,  7,  329). 

The  concentration  of  the  solution  is,  of  course,  increased  by 
the  freezing  of  a  portion  of  the  solvent.  This  explains  the 
falling  of  the  thermometer  which  is  observed  to  follow  its  rise 
when  freezing  begins.  Since  the  quantity  of  ice  which  will  be 
formed  is  determined  by  the  amount  of  the  under-cooling,  the 
error  due  to  such  an  increase  in  concentration  may  be  dimin- 
ished by  cooling  the  solution  only  slightly  below  its  freezing 
point.  The  increase  in  concentration  may  be  calculated  by  the 
formula 

r  =  — »  in  which 

A» 

r  is  the  fraction  of  the  solvent  which  solidifies  in  consequence 
of  the  under-cooling, 

0  the  degree  of  the  under-cooling  which  preceded  solidifica- 
tion, 

c  the  specific  heat  of  the  solvent,  and 

X  the  heat  of  solidification  of  a  unit  quantity  of  the  solvent. 


DETERMINATION  OF  MOLECULAR  WEIGHTS         179 

2.  CANE  SUGAR 

Proceed  as  directed  under  1,  using  water  as  the  solvent  and 
"  rock  candy  "  as  the  material  to  be  experimented  upon. 

3.  CHLOROFORM 

The  glacial  acetic  acid  which  is  used  as  the  solvent  in  this 
experiment  must  be  carefully  protected  from  the  moisture  of 
the  air.  It  is  advisable  to  use  the  apparatus  represented  in 
Fig.  37,  and  to  conduct  air  dried  by  sulphuric  acid  through  G. 
Glacial  acetic  acid,  when  pure,  melts  at  16.75°.  It  is,  however, 
hygroscopic  and  is  therefore  usually  found  to  fuse  at  a  lower 
temperature.  The  presence  in  it  of  a  small  quantity  of  water 
does  not  interfere  with  its  use  as  a  solvent,  provided  the  same 
is  not  acquired  during  the  course  of  an  experiment. 

Glacial  acetic  acid  gives  normal  results  with  nearly  all  sub- 
stances which  dissolve  in  it  without  chemical  action.  The 
molecular  depression  of  its  freezing  point  is  39. 

THE  BOILING-POINT  METHOD 

If  a  nonvolatile  substance  is  dissolved  in  a  volatile  liquid,  the 
vapor  tension  of  the  latter  is  diminished  and  its  boiling  point  is 
therefore  raised.  The  effects  produced  by  substances  in  solution 
upon  the  vapor  tension  and  boiling  points  of  liquids  are  in  all 
respects  analogous  to  those  produced  upon  their  freezing  points. 

In  the  case  of  a  given  solvent  and  soluble  substance,  the 
lowering  of  the  vapor  tension  at  any  given  temperature  and 
the  consequent  elevation  of  the  boiling  point  are  proportional 
to  the  concentration  of  the  solution. 

Again,  equi-molecular  quantities  of  different  substances,  when 
dissolved  in  the  same  weight  of  any  given  liquid,  produce  an 
equal  lowering  of  the  vapor  tension,  and  consequently  an  equal 
elevation  of  the  boiling  point  of  the  solvent. 

No  practicable  method  has  yet  been  devised  for  the  determi- 
nation of  molecular  weights  by  measuring  directly  the  effect  of 


180  QUANTITATIVE  EXERCISES 

substances  in  solution  upon  the  vapor  tension  of  the  solvents.  It 
has  been  found  easier  to  utilize  for  this  purpose  the  temperatures 
of  equal- vapor  tension,  i.e.  the  boiling  points  of  the  solvents. 

The  definitions  employed  are  similar  to  those  which  were 
given  in  connection  with  the  freezing-point  method. 

1.  The  specific  elevation  of  the  boiling  point  of  any  liquid  is 
that  which  is  observed  when  1  gram  of  a  substance  is  dissolved 
in  100  grams  of  the  solvent.     Obviously  the  specific  elevations 
produced  by  different  substances  are  inversely  proportional  to 
the  molecular  weights  of  the  substances. 

2.  The  molecular  elevation  of  the  boiling  point  of  a  liquid  is 
that  which  is  produced  when  a  number  of  grams  of  a  substance 
equal  to  its  molecular  weight  is  dissolved  in  100  grams  of  the 
solvent.     In  the  case  of  any  given  substance,  the  molecular  ele- 
vation is  equal  to  the  product  of  the  specific  elevation  and  the 
molecular  weight  of  the  substance.     It  is  a  constant  for  any 
given  solvent  in  the  same  way  and  with  the  same  limitations  as 
the  molecular  depression  of  the  freezing  point.     Like  the  latter, 
it  may  be  determined  experimentally  by  operating  with  weighed 
quantities  of  the  liquid  and  of  some  substance  of  known  molec- 
ular weight,  or  it  may  be  calculated  by  means  of  the  formula 

7^2 
(7=0.02  — ,  in  which 

C  is  the  constant  to  be  found, 

T  the  absolute  boiling  point  of  the  liquid,  and 

W  its  latent  heat  of  vaporization. 

The  following  table  gives  the  molecular  elevation  of  the  boil- 
ing points  of  several  liquids. 

Ether 21.1  Benzene 26.7 

Acetone 16.7  Water 5.2 

Chloroform .     .     .     .  36.6  Acetic  acid  ....  25.3 

Carbon  bisulphide      .  23.7  Ethylene  bromide      .  63.2 

Alcohol 11.5  Phenol 30.4 

Ethyl  acetate    .     .     .  26.1  Aniline 32.2 

Methyl  alcohol      .     .  9.2  Isopropyl  alcohol  .     .  12.9 


DETERMINATION  OF  MOLECULAR  WEIGHTS         181 

If  the  molecular  elevation  of  any  solvent  is  divided  by  the 
molecular  weight,  the  quotient  is  the  elevation  of  the  boiling 
point  which  one  molecule  of  any  substance,  not  dissociating  in 
solution,  will  produce  in  100  molecules  of  the  solvent. 

The  method  of  deducing  the  molecular  weight  of  a  substance 
from  the  effect  which  it  produces  on  the  boiling  point  of  a  liquid 
is  the  same  as  that  employed  in  the  freezing-point  process. 
If  g  grams  of  the  substance  produce  an  elevation  e  in  G  grams 
of  the  solvent,  then  the  quantity  of  the  substance  which  will 
produce"  the  same  elevation  in  100  grams  of  the  solvent  is 

100   g  ;  for  x  :  100  :  :  g  :  G.     Again,  if  100  $-  grams  of  the 
G  G 

substance  produce  the  elevation  e  in  100  grams  of  the  solvent, 
the  quantity  which  will  produce  the  molecular  elevation  is 

100  C-&-,  since 
Ge 


G  Ge 

m  represents  the  molecular  weight  of  the  substance,  and  its 
value  is  found  by  substituting  in  the  formula  the  numerical 
values  of  (7,  #,  G,  and  e. 

As  might  be  expected  from  their  behavior  when  tested  by 
the  freezing-point  method,  the  class  of  bodies  known  as  electro- 
lytes gives,  when  dissolved  in  water,  abnormal  elevations  of  the 
boiling  point. 

In  benzene,  chloroform,  and  carbon  bisulphide,  certain  bodies, 
especially  hydroxyl  compounds,  give  less  than  the  calculated 
elevation  of  the  boiling  point.  This  is  supposed  to  be  due  to 
the  existence  in  such  solutions  of  complex  molecules,  consisting 
either  of  the  dissolved  substance  and  the  solvent,  or  of  the 
former  only.  The  results,  however,  become  more  nearly  nor- 
mal with  increasing  dilution.  When  a  substance  of  unknown 
molecular  weight  is  to  be  investigated,  it  is  safer  to  test  its 
effect  upon  the  boiling  points  of  several  liquids. 


182 


QUANTITATIVE  EXERCISES 


With  reference  to  the  selection  of  a  solvent  for  any  particu- 
lar determination,  it  is  to  be  observed  that  the  liquid  chosen 
should  be  one  in  which  the  substance  is  very  readily  soluble, 
also  that  its  boiling  point  should  be,  in  general,  not  less  than 
130°  or  140°  lower  than  that  of  the  substance  to  be  investi- 
gated. If  the  boiling  points  of  the  substance  and  of  the  avail- 
able solvents  are  too  near  together,  the  freezing-point  rather  than 
the  boiling-point  method  should  be  employed. 
As  a  rule,  the  best  results  are  obtained  when 
ether,  alcohol,  ethyl  acetate,  acetone,  acetic 
acid,  formic  acid,  thymol,  and  phenol  are  used 
as  solvents. 

A  number  of  forms  of  apparatus  for  the 
determination  of  molecular  weights  by  the 
boiling-point  method  have  been  devised  and 
~  recommended.  Only  one  of  them,  that  of 
H.  C.  Jones,  Fig.  39,  will  be  described  here. 
It  consists  of  a  glass  tube  A,  180  mm.  in 
length  and  40  mm.  in  diameter,  to  the  side 
of  which  is  attached  the  smaller  tube  a.  It  is 
contracted  at  the  top  to  a  diameter  of  about 
27  mm.  arid  ground  to  receive  a  light  glass 
stopper,  which  is  inserted  when  the  apparatus 
is  to  be  weighed.  The  tube  is  filled  with 
glass  beads  to  a  height  of  30  or  40  mm.,  and 
into  these  is  pressed  to  a  depth  of  15  or  20  mm. 
the  platinum  cylinder  P,  which  has  a  length 
of  80  mm.  and  a  diameter  of  25  mm.  The 
space  within  the  platinum  cylinder,  under  and  around  the  bulb 
of  the  thermometer,  is  filled  with  scraps  of  platinum  foil  whose 
corners  have  been  bent  in  different  directions  to  prevent  too 
close  packing  and  whose  edges  have  been  serrated  to  facilitate 
the  escape  of  vapor.  The  condenser  tube  which  enters  at  a  is 
provided  at  the  top  with  a  U-tube  containing  some  drying  agent. 
The  apparatus  is  surrounded  by  a  jacket  of  asbestus  Jf,  120  mm. 


FIG.  39 


DETERMINATION  OF  MOLECULAR  WEIGHTS         188 

in  height  and  15  mm.  in  thickness,  which  rests  on  an  asbestus 
board  having  a  hole  in  the  center.  The  hole  is  covered  with 
a  piece  of  wire  gauze. 

EXERCISE  XV 

DETERMINATION   OF   MOLECULAR  WEIGHTS   BY   THE 
BOILING-POINT   METHOD 

1.  IODINE 

Adjust  the  thermometer  so  that  the  upper  end  of  the  mer- 
cury column  will  be  in  the  lower  part  of  the  scale  at  the  boiling 
point  of  ether.  Carefully  cleanse  and  dry  the  apparatus, 
especially  those  parts  which  will  afterwards  come  in  contact 
with  the  solvent,  and  arrange  the  beads,  platinum  cylinder, 
and  platinum  scraps  as  indicated  in  the  description  of  the 
apparatus.  Close  A  with  the  ground-glass  stopper  and  the 
side  tube  a  with  an  ether-tight  cork  and  weigh.  Introduce 
somewhat  more  anhydrous  ether  than  is  necessary  to  cover  the 
bulb  of  the  thermometer  when  in  place  and  weigh  again.  If  a 
balance  of  sufficient  capacity  to  permit  the  weighing  of  so  heavy 
a  load  is  not  available,  the  solvent  must  be  weighed  separately. 
Arrange  everything  as  shown  in  the  figure;  heat  cautiously 
and  determine  the  boiling  point  of  the  ether.  Several  observa- 
tions should  be  taken  at  considerable  intervals  of  time,  and  the 
height  of  the  barometer  should  be  recorded  with  each  observa- 
tion of  the  temperature.  The  thermometer  must  be  vigorously 
jarred  just  before  reading,  and  the  reading  should  be  made  after 
a  rise  rather  than  after  a  fall  of  temperature.  This  may  always 
be  done  by  allowing  the  liquid  to  cool  a  little  and  then  reheat- 
ing it  to  its  boiling  point. 

Introduce  through  the  side  tube,  when  the  ether  is  cold, 
from  0.5  to  0.7  gram  of  dry  resublimed  iodine,  and  again  deter- 
mine the  boiling  point,  recording,  as  before,  the  height  of  the 
barometer.  Afterwards  introduce  several  other  small  weighed 


184  QUANTITATIVE  EXERCISES 

portions  of  iodine,  from  0.1  to  0.2  gram,  determining  the  boil- 
ing point  after  each  addition.  Finally,  when  the  apparatus  has 
cooled,  weigh  again  in  order  to  ascertain  whether  any  ether  has 
been  lost  during  the  experiment. 

Calculate  the  molecular  weight  of  iodine  from  the  results  of 
the  individual  determinations  and  from  the  total  observed  ele- 
vation of  the  boiling  point,  using  21.1  as  the  molecular  eleva- 
tion of  the  boiling  point  of  ether.  Calculate  also  from  the 
observed  elevation  and  the  known  molecular  weight  of  iodine 
the  molecular  elevation  of  the  boiling  point  of  the  solvent. 

2.  NAPHTHALENE 

Proceed  exactly  as  directed  under  1. 

The  boiling  solution  has  something  more  than  its  calculated 
concentration,  owing  to  the  absence  of  a  portion  of  the  solvent 
in  the  form  of  vapor  and  of  returning  liquid.  Errors  from  this 
cause  are  reduced  to  a  minimum  by  carefully  regulating  the 
rate  of  boiling,  which  should  not  be  more  rapid  than  is  neces- 
sary for  the  maintenance  of  a  constant  temperature  and  to 
prevent  the  overheating  of  the  liquid. 

The  temperature  indicated  by  the  thermometer  varies  some- 
what according  to  the  depth  to  which  the  bulb  is  submerged  in 
the  boiling  liquid.  It  is  therefore  well  to  maintain  the  same 
degree  of  submergence  through  the  whole  course  of  any  single 
determination  of  a  molecular  weight. 

The  most  serious  difficulty  which  is  encountered  in  the  use 
of  the  boiling-point  method  is  the  variability  of  atmospheric 
pressure.  A  rise  or  fall  of  one  millimeter  in  the  barometer  is 
followed  by  a  change  of  boiling  point  amounting  in  the  case  of 
most  liquids  to  nearly  .03°.  It  is  obvious  that  the  real  results 
of  a  determination  may  be  wholly  masked  in  consequence  of 
such  barometric  fluctuations  as  frequently  occur  within  the 
time  required  for  an  experiment.  This  difficulty  may  be  met 
more  or  less  successfully  in  various  ways. 


DETERMINATION   OF  MOLECULAR  WEIGHTS         185 

1.  A  second  apparatus  containing  the  solvent  only  may  be 
placed  beside  that  in  which  the  determination  is  made.     If  the 
liquids  in  both  are  boiled  under  the  same  conditions,  as  nearly 
as  possible,  and  if  the  two  thermometers  are  read  simultane- 
ously, the  observed  fluctuations  of  the  boiling  point  of  the  pure 
solvent  may  be  applied  as  corrections  to  the  observed  elevations 
of  the  boiling  points  of  the  solution. 

2.  A  pressure  regulator  may  be  employed  (see  Zeitschrift  fur 
physikalische  Chemie,  11,  25). 

3.  All  observed  boiling  points  may  be  corrected  to  standard 
pressure  by  means  of  the  formula 

To 

Correction  =  ±  n  —  ,  in  which 
oO 

T  is  the  absolute  boiling  point  of  the  liquid  under  standard 
pressure, 

c  a  constant  depending  upon  the  chemical  nature  of  the 
solvent,  and 

n  the  difference  in  millimeters  between  760  and  the  observed 
height  of  the  barometer. 

The  value  of  c  for  several  different  liquids,  also  the  meaning 
of  the  formula  and  the  manner  of  using  it,  will  be  found  on 
page  191  of  the  second  edition  of  Landolt  and  Boernstein's 
Physikalisch-  Chemische  Tabellen. 


CHAPTER  VIII 
THE  PURIFICATION  OF   SUBSTANCES 

The  preparation  of  substances  in  pure  condition  and  the 
maintenance  of  their  purity  are  obviously  matters  of  the  highest 
importance  in  quantitative  chemistry ;  nevertheless,  but  little 
of  a  comprehensive  character  which  is  likely  to  be  useful  to 
the  student  can  be  said  upon  these  subjects.  The  processes  of 
purification  and  the  measures  adopted  to  prevent  contamination 
must  be  constantly  modified  to  suit  the  conditions,  i.e.  the  char- 
acter of  the  substances  to  be  purified  and  the  nature  of  the 
impurities  to  be  removed  or  guarded  against.  Hence  a  minute 
knowledge  of  the  reactions  of  the  substances  concerned,  rather 
than  of  general  rules,  and  vigilance  are  essential  to  a  judi- 
cious selection  of  methods  and  a  successful  execution  of  them. 
There  are,  however,  a  few  operations,  such  as  evaporation,  recrys- 
tallization,  filtration,  washing  upon  filters,  etc.,  which  are  so 
frequently  employed  in  the  preparation  of  materials  in  pure 
condition  that  some  observations  regarding  them  may  be  of 
advantage. 

1.  EVAPORATION  OF  LIQUIDS 

When  a  solution  is  to  be  evaporated  for  the  purpose  of 
increasing  its  concentration  or  for  the  purpose  of  purifying  a 
substance  by  recrystallization,  attention  should  be  paid  to  the 
possible  action  of  the  solvent  and  the  dissolved  matter  upon 
the  material  of  the  containing  vessel.  In  general,  a  porcelain 
vessel  is  less  readily  attacked  by  reagents  than  one  of  glass, 
and  platinum  resists  their  action  still  better  than  porcelain. 
Glass  and  porcelain  are  much  more  readily  attacked  by  alkaline 
than  by  neutral  or  acid  solutions,  and  less  readily  by  those  which 

186 


THE  PURIFICATION  OF  SUBSTANCES  187 

are  moderately  acid  than  by  those  which  are  neutral.  Hence,  as  a 
rule,  if  such  a  course  is  practicable,  an  alkaline  or  even  a  neutral 
solution  which  is  to  be  concentrated  in  glass  or  porcelain  should 
be  slightly  acidified.  This  may  often  be  done  without  disadvan- 
tage when  the  substance  whose  purity  is  an  object  of  concern  is 
a  salt,  since  the  presence  in  a  solution  of  a  slight  excess  of  the 
same  acid  as  that  in  the  salt  is  frequently  unobjectionable. 

Most  of  the  glass  used  in  a  chemical  laboratory  is  essentially 
a  silicate  of  the  alkali  metals,  potassium  and  sodium,  and  of  cal- 
cium. The  character  of  the  glass,  especially  as  regards  its  power 
to  resist  the  action  of  reagents,  depends  upon  the  ratio  of  these 
three  constituents,  and  apparently  more  upon  the  ratio  of  the 
alkaline  and  alkaline-earth  bases  to  each  other  than  upon  that 
of  the  silica  to  the  sum  of  the  bases.  An  increase  in  the  pro- 
portion of  the  calcium  improves  the  glass  in  respect  to  its  ability 
to  resist  the  action  of  reagents,  but  at  the  same  time  increases  the 
difficulty  of  working  it  in  the  flame.  An  increase  in  the  pro- 
portion of  the  alkalies  produces  precisely  the  opposite  effects ; 
that  is,  glass  containing  a  relatively  high  percentage  of  the  alka- 
lies is  easily  worked,  and  is  also  readily  attacked  by  reagents. 
The  injurious  effect  of  a  large  proportion  of  the  alkalies  may 
however  be  diminished  to  some  extent  by  increasing  the  propor- 
tion of  the  silica.  A  soda  glass  is  said  to  resist  the  action  of 
reagents  better  than  a  potash  glass  of  analogous  composition. 

Two  varieties  of  glass  of  established  excellence  for  chemical 
purposes  have  been  put  upon  the  market.  One  of  these,  the  so- 
called  Stas  glass,  is  characterized  by  its  great  resisting  power, 
and  also,  unfortunately,  by  the  difficulty  of  working  it  in  the 
flame.  Its  composition  is  represented  by  the  ratio 

(K20,  Na20) :  CaO  :  SiO2  : :  1  :  1  :  7. 

The  second  variety,  known  as  Weber's  glass,  has  also  a  considerable 
resisting  power  and  is  worked  with  a  fair  degree  of  ease.  Its 
composition  is  represented  by  the  ratio 

(K20,  Na20)  :  CaO  ;  SiO2  : :  L37  :  1  :  7.3, 


188  QUANTITATIVE  EXERCISES 

A  third  variety  of  glass  known  as  "  the  Jena  utensil  and  tube 
glass  for  chemical  and  physical  uses  "  has  within  recent  years 
entered  into  competition  with  the  long-known  and  highly  valued 
Bohemian  or  "hard"  glass.  According  to  one  certificate  of 
official  examination,  this  glass  was  found  to  excel  the  best 
Bohemian  in  the  following  ratios : 

4  to  5  :  1,  when  treated  with  water  at  20°, 
11  to  12  :1  "          "          "         "      "  80°, 

3:1"  "  "     2-normal  soda  solution. 

When  treated  for  6  hours  with  a  normal  solution  of  sulphuric 
acid  at  100°,  neither  glass  was  sensibly  attacked.  On  the  other 
hand,  as  regards  resistance  to  the  action  of  a  2-normal  solution 
of  sodium  hydroxide,  the  Bohemian  was  found  to  exel  the  Jena 
glass  in  the  ratio  of  3  :  2.  The  Jena  glass  is  said  to  be  greatly 
superior  to  the  Bohemian  in  respect  to  its  power  to  withstand 
sudden  changes  of  temperature. 

The  question  of  the  composition  of  the  glass  employed  for  chemical  purposes, 
and  the  relation  of  its  qualities,  good  and  bad,  to  composition  is  worthy  of  much 
more  general  and  systematic  attention  in  the  laboratory  than  has  hitherto  been 
given  to  it.  The  following  references  to  published  communications  may  be  of 
use  to  those  who  wish  to  learn  what  observations  upon  this  subject  have  already 
been  made  public. 

R.  Fresenius,  Quantitative  Analyse,  2,  796. 

A.  Emmerling,  Liebig's  Annalen,  150,  257  (Zeitschrift  fur  analytische  Chemie, 
8,  434). 

W.  Fresenius,  Zeitsch.  anal.  Chem.,  22,  397. 

E.  Bohlig,  Zeitsch.  anal.  Chem.,  23,  518. 

Kreusler  and  Henzold,  Berichte  der  deutschen  chemischen  Gesellschaft,  17, 

34  (Zeitsch.  anal.  Chem.,  23,  532). 
V.  Wartha,  Zeitsch.  anal.  Chem.,  24,  220. 
Marshall  and  Potts,  American  Chemical  Journal,  10,  425  (Zeitsch.  anal. 

Chem.,  28,  613). 
Stas,  Chemical  News,  17,  1. 
R.  Weber,  and  R.  Weber  and  E.  Sauer,  Annalen  der  Physik  und  der  Chemie 

(N.F.),  4,  431. 

Zeitschrift  fur  angewandt    Chemie,  1891,  662. 
Ber.  d.  d.  chem.  Ges.,  25,  70,  1814. 
Zeitsch.  anal.  Chem.,  31,  425,  672. 

F.  Mylius,  Ber.  d.  d.  chem.  Ges,  22,  310. 


THE  PURIFICATION  OF   SUBSTANCES  189 

F.  Mylius,  Zeitsch.  anal.  Chem.,  30,  247. 

O.  Schott,  Zeitschrift  fur  Instrumentenkunde,  9,  81;  11,  331  (Zeitsch.  anal. 

Chem.);  30,  317;  31,  419. 
F.  Mylius  and  F.  Foerster,  Zeitsch.  fur  Instrumentenkunde,  9,  117  (Zeitsch. 

anal.  Chem.,  30,  317). 
Zeitsch.  anal.  Chem.,  31,    24. 
F.  Kohlrausch,  Ann.  der  Phys.  und  Chem.  (N.F.),  44,  577  (Zeitsch.  anal. 

Chem.,  31,  421). 
Ber.  d.  d.  chem.  Ges.,  24,  3560;  26,  2998  (Zeitsch.  anal.  Chem.,  34,  591). 

E.  Pfeiffer,  Ann.  der  Phys.  und  Chem.  (N.F.),  44,  239. 

F.  Foerster,  Zeitsch.  anal.  Chem.,  33,  299,  322,  381. 

A.  Winkelmann  and  0.  Schott,  Zeitsch.  fur  Instrumentenkunde,  14,  6  (Zeitsch. 
anal.  Chem.,  34,  591). 

The  danger  that  exposed  solutions  will  become  contaminated 
by  the  dust  in  the  air  and  by  the  volatile  reagents  which  are 
used  in  the  laboratory  is  too  obvious  to  require  more  than  a 
passing  mention.  There  is,  however,  one  source  of  contamina- 
tion which  is  not  so  generally  recognized  as  it  deserves  to  be. 
It  is  the  sulphur  in  the  illuminating  gas.  Any  aqueous  solu- 
tion or  basic  substance  which  is  heated  in  an  open  vessel  over 
a  lamp  burning  illuminating  gas  is  in  danger  of  becoming  con- 
taminated with  sulphuric  acid  unless  precautions  are  taken  to 
divert  the  products  of  combustion.  For  this  reason  it  is  often 
better  to  evaporate  liquids  and  to  heat  strongly  basic  substances 
over  an  alcohol  lamp  rather  than  over  a  Bunsen  burner. 

The  so-called  bumping  of  evaporating  solutions  is  frequently 
a  source  of  trouble.  The  cause  of  the  phenomenon  is  the 
superheated  condition  of  the  liquid  in  that  part  of  the  vessel 
to  which  the  heat  is  applied,  usually  the  bottom,  which  in- 
creases to  a  certain  degree,  when  a  large  amount  of  vapor 
suddenly  forms  and  escapes  through  the  liquid  above  with 
explosive  violence.  In  the  case  of  actively  boiling  liquids  it 
is  frequently  helpful  to  introduce  scraps  of  platinum  foil  with 
serrated  edges,  or  bits  of  broken  glass,  or  anything  in  fact 
which,  owing  to  its  numerous  sharp  edges  and  points,  facilitates 
the  escape  of  vapor  from  a  hot  liquid.  The  disturbance  usually 
ceases  if  the  vessel  is  placed  in  a  bath  of  any  kind  so  that  the 


190  QUANTITATIVE  EXERCISES 

liquid  is  heated  uniformly  upon  all  sides.  Hence  the  utility  of 
flasks  surrounded  with  a  net  of  woven  wire,  and  of  metallic 
vessels  for  the  evaporation  of  liquids  which  exhibit  this  propen- 
sity. If  a  liquid  is  to  be  rapidly  evaporated  in  an  open  metallic 
vessel,  e.g.  a  platinum  crucible  or  dish,  the  vessel  should  be 
tilted  to  one  side  and  the  flame  applied  to  it  above  the  surface 
of  the  liquid  instead  of  below  it.  Any  arrangement  is  advan- 
tageous which  enables  one  to  apply  the  heat  principally  to  the 
upper  portions  of  an  evaporating  liquid,  or  at  least  as  much  to 
the  upper  as  to  the  lower  portions.  A  bath  which  is  service- 
able when  a  liquid  of  not  too  high  boiling  point  is  to  be  evapo- 
rated in  a  beaker  or  other  vessel  of  similar  form  is  quickly  made 
in  the  following  manner:  A  long  strip  of  copper  wire  gauze, 
whose  width  is  somewhat  greater  than  the  height  of  the  vessel, 
is  tightly  wound  several  times  around  the  beaker  and  fastened 
with  wire.  The  lower  edges  are  turned  inward  until  they 
meet,  and  hammered  into  a  flat  and  compact  bottom.  The  bath 
is  filled  to  a  depth  of  10  or  15  mm.  with  loose  asbestus,  and  its 
vertical  outside  covered  with  several  thicknesses  of  asbestus 
paper.  It  is  best  heated  on  an  iron  plate.  A  simple  but  often 
useful  device  for  applying  heat  to  a  low-boiling  liquid  at  any 
desired  point  is  a  copper  wire  which  is  wound  around  the  con- 
taining vessel.  The  ends  of  the  wire  are  allowed  to  extend 
to  a  safe  distance  from  the  apparatus  and,  singly  or  twisted 
together,  are  heated  with  a  lamp  or  by  other  suitable  means. 
The  arrangement  is  more  effective  if  the  surface  to  be  wound 
with  the  wire  is  first  covered  with  copper  foil. 


,   2.  RECRYSTALLIZATION 

When  a  hot  saturated  solution  of  a  substance  is  cooled  for  the 
purpose  of  purifying  the  substance  by  recrystallization,  the  liquid 
should  be  stirred  while  the  crystals  are  forming.  In  this  way 
small  crystals  are  obtained  which  are  less  likely  to  inclose  mother 
liquor  than  the  large  ones  which  form  in  tranquil  solutions. 


THE    PURIFICATION    OF   SUBSTANCES  191 

Crystals  collected  upon  paper  filters  are.  liable  when  removed 
to  become  contaminated  with  shreds  of  the  paper.  It  is  there- 
fore better,  as  a  rule,  to  collect  the  product  of  a  recrystalliza- 
tion  in  a  funnel  in  the  bottom  of  which  only  a  platinum  cone 
or  a  perforated  porcelain  disk  has  been  placed.  If  the  nitrate 
is  found  to  contain  too  many  crystals,  it  may  be  again  passed 
through  the  filter. 

It  is  important,  after  each  recrystallization,  to  remove  from 
the  crystals  as  completely  as  possible  the  adhering  mother 
liquor.  To  this  end  a  filter  pump  should  be  employed,  and 
the  mass  in  the  funnel  should  be  made  as  compact  as  possible 
by  pressing  and  stamping  with  a  pestle  or  other  hard  object. 
Finally,  when  no  more  liquid  can  be  extracted  by  means  of  the 
pump,  the  crystals  should  be  immediately  transferred  to  a  clean 
unglazed  porcelain  plate  and  then  stirred  until  they  are  thor- 
oughly dry.  If  the  substance  is  not  too  soluble,  the  crystals, 
after  removal  of  the  mother  liquor  by  the  pump,  may  be  mois- 
tened with  pure  water,  allowed  to  stand  for  a  short  time,  and 
again  freed  from  mother  liquor  by  pumping.  In  some  cases 
the  adhering  mother  liquor  may  be  removed  by  washing  the 
crystals  with  a  liquid,  e.g.  alcohol,  in  which  they  are  insoluble, 
or  only  slightly  soluble. 

Many  substances  containing  water  of  crystallization,  such  as 
CuSO4.5H2O,  C2O4H2.2H2O,  etc.,  which  appear  to  be  stable 
in  the  air,  really  exert  a  vapor  tension  at  ordinary  temperatures, 
as  is  proved  by  their  efflorescence  when  placed  in  desiccators. 
Such  substances  can  only  be  air-dried,  and  that  only  at  ordinary 
temperatures,  or  dried  over  desiccating  materials  which  have  a 
vapor  tension  equal  to  or  greater  than  their  own. 

The  handling,  in  quantitative  operations,  of  substances  hav- 
ing water  of  crystallization  or  capable  of  acquiring  it  from  a 
moist  atmosphere  requires  intelligent  management.  One  gen- 
eral rule,  however,  may  be  given  with  respect  to  their  treatment, 
namely,  that  substances  which  contain  water  of  crystallization, 
but  are  not  hygroscopic,  must  at  all  times  be  surrounded  by  an 


192  QUANTITATIVE  EXERCISES 

atmosphere  in  which  the  tension  of  water  vapor  is  sufficient  to 
prevent  their  dissociation,  while  substances  which  are  capable 
of  absorbing  water  must  be  kept  in  an  atmosphere  in  which  the 
tension  of  water  vapor  is  just  equal  to  their  own.  It  will  be 
readily  understood  that  it  is  not  often  practicable  to  conform 
to  the  latter  condition  if  the  hygroscopic  substances  contain  any 
water  whatsoever;  hence  such  materials,  as  a  rule,  must  be 
completely  freed  from  water  and  preserved  thereafter  in  a  dry 
atmosphere.  This  course,  however,  often  leads  to  difficulties, 
especially  in  the  case  of  salts,  many  of  which,  when  heated  to  the 
temperatures  requisite  for  their  complete  dehydration,  decompose 
with  loss  of  acid  and  formation  of  more  or  less  of  basic  salts.* 


3.  DESICCATION 

In  drying  substances  to  a  constant  weight  we  have  to  deal 
with  hygroscopic  moisture,  i.e.  the  water  which  condenses  on  the 
surfaces  of  all  solids  which  are  exposed  to  a  moist  atmosphere ; 
with  the  water  which  adheres  to  the  outside  of  bodies  when 
they  are  separated  from  liquids,  as  by  filtration ;  with  water  of 
crystallization  ;  and,  less  frequently,  with  so-called  water  of  con- 
stitution, or  chemically  combined  water.  In  all  of  these  cases 
the  desiccation  is  effected  by  means  of  drying  agents  or  by  heat, 
or  by  means  of  both  drying  agents  and  heat. 

To  desiccate  a  substance  by  means  of  a  drying  agent,  it  is 
placed  with  the  absorbent  in  a  closed  vessel  (a  desiccator),  care 
being  taken  to  provide  for  the  greatest  possible  freedom  of  dif- 
fusion between  them.  Since  the  evaporation  of  the  water  upon 
or  within  the  substance  to  be  dried  must  precede  its  absorp- 
tion, anything  which  promotes  its  volatilization  is  of  advantage. 

*  It  is  unfortunate  that  we  have  not,  at  the  present  time,  a  more  extensive 
and  systematic  knowledge  of  the  aqueous  vapor  tension  exerted  at  different 
temperatures  by  substances  containing  water,  whether  of  crystallization  or  of 
constitution,  and  by  aqueous  solutions  of  varying  concentration,  since  such 
knowledge,  aside  from  its  value  in  other  directions,  would  be  of  the  greatest 
utility  in  many  quantitative  operations. 


THE  PURIFICATION  OF  SUBSTANCES  193 

Hence  desiccators  are  often  attached  to  filter  pumps  for  the 
purpose  of  diminishing  the  pressure  within  them.  Sometimes, 
with  the  same  end  in  view,  they  are  heated.  This  is  permis- 
sible in  some  cases  but  not  in  all.  If  the  absorbent,  like  cal- 
cium chloride,  has  a  considerable  vapor  tension  of  its  own  at 
elevated  temperatures,  the  desiccators  must  be  kept  cool.  The 
drying  agents  which  are  commonly  employed  in  desiccators  are 
those  that  were  mentioned  in  connection  with  the  drying  of 
gases,  such  as  calcium  chloride,  sulphuric  acid,  and  phosphorus 
pentoxide ;  and  since  there  is  no  essential  difference  between 
the  drying  of  a  gas  when  it  is  passed  over  an  absorbent  and 
the  drying  of  a  solid  when  it  is  inclosed  with  the  absorbent  in 
a  desiccator,  whatever  was  said  of  them  in  the  former  connec- 
tion is  equally  applicable  to  them  in  the  latter.  It  may,  how- 
ever, be  well  to  recall  one  caution,  namely,  that  when  sulphuric 
acid  is  used  as  a  drying  agent,  care  must  be  taken  to  exclude 
organic  matter  and  all  other  substances  which  act  upon  it  with 
formation  of  sulphur  dioxide. 

The  evaporation  of  liquids  at  temperatures  below  their  boil- 
ing points  is  greatly  facilitated  by  constantly  replacing  the  sat- 
urated atmosphere  about  them  by  one  which  is  unsaturated 
with  the  vapor  of  the  liquids.  This  principle  can  often  be 
utilized  with  great  advantage,  not  only  in  the  evaporation  of 
masses  of  liquids,  but  also  in  the  desiccation  of  solids  both  at 
ordinary  and  at  more  elevated  temperatures.  The  most  effi- 
cient of  all  desiccators  is  one  through  which  a  current  of  dry 
gas,  e.g.  air,  is  passing.  Conditions  which  retard  the  diffusion 
of  the  water  vapor  away  from  the  drying  materials  should,  of 
course,  be  avoided.  For  this  reason  substances  to  be  dried, 
like  liquids  to  be  evaporated,  should  be  placed  in  shallow  ves- 
sels rather  than  in  those  with  high  sides.  It  will  be  seen,  too, 
that  advantage  is  to  be  gained  by  increasing  the  area  of  the 
evaporation,  i.e.  by  spreading  the  material  to  be  desiccated. 

The  removal  of  liquids  from  the  surfaces  of  solids  by  evapo- 
ration, as  in  the  drying  of  precipitates,  is  much  slower  at  any 


194 


QUANTITATIVE  EXERCISES 


given  temperature  than  could  be  anticipated  from  the  known 
volatility  of  the  liquids  at  that  temperature.  This  is  due  to 
the  fact  that  the  vapor  tension  of  a  liquid,  when  in  the  form 
of  a  film  upon  the  surface  of  a  solid,  is  less  at  a  given  tem- 
perature than  in  other  situations ;  also,  in  many  instances,  to 
the  fact  that  such  films  are,  in  reality,  concentrated  solutions 
of  the  material  of  the  solids  which  they  surround.  It  is  there- 
fore well,  if  practicable,  when  a  wet  substance  is  to  be  rapidly 
dried,  to  heat  it  to  a  temperature  several  degrees  above  the 
boiling  point  of  the  liquid  to  be  removed. 

Hot-Air  Baths 

The  ordinary  rectangular  copper  hot-air  bath  is  too  familiar 
an  object  in  the  laboratory  to  require  description.     This  form 
of  bath,  though  convenient  and  in  universal  use,  is  sometimes 
a     unsuited  to  the  >  dry  ing  of  materials,  owing  to  the 
readiness  with  which  it  is  entered  by  the  prod- 
ucts of  combustion  from  the  lamp  beneath.     In 
this  respect  the  cylindrical  bath  with  its  tight 
bottom  and  removable  cover  is  superior  to  the 
usual  form. 

The  maintenance  of  constant  temperatures  in 
baths  is  a  matter  of  great  importance,  and  a  large 
number  of  automatic  gas  regulators  have  been 
devised  for  this  purpose.  The  best  of  these  for 
general  use  is  probably  the  Reichert  regulator 
shown  in  Fig.  40.  The  gas  enters  the  hollow 

O  O 

stopper  at  a  and  passes  through  b  to  the  lamp 
under  the  bath,  one  portion  escaping  from  the 
stopper  through  the  small  hole  <?,  and  the  remain- 
der through  the  lower  contracted  end  d.  The 
hole  c  is  so  related  to  the  outer  wail  that  the  flow  of  gas  through 
it  may  be  regulated,  but  not  entirely  stopped,  by  turning  the 
stopper  to  the  right  or  to  the  left.  The  flow  through  d,  on 
the  other  hand,  may  be  entirely  cut  off  by  the  expansion  of 


FIG.  40 


THE  PURIFICATION  OF  SUBSTANCES  195 

the  mercury  in  the  bulb  e.  The  instrument  is  provided  with 
a  side  tube  and  a  screw/",  by  means  of  which  the  column  of  mer- 
cury in  g  can  be  brought  to  any  desired  height.  The  adjustment 
of  the  instrument  for  the  maintenance  of  any  desired  tempera- 
ture is  effected  in  the  following  manner:  The  screw /is  turned 
to  the  right  until  the  mercury  rises  and  closes  the  passage  d ;  the 
flow  of  gas  through  c  is  then  regulated  by  turning  the  stopcock 
to  the  right  or  left  until  a  flame  is  produced  which  will  give  to 
the  bath,  under  all  probable  changes  in  the  pressure  on  the  gas 
supply,  a  temperature  somewhat  below  that  required.  The 
screw /is  now  turned  to  the  left,  and  gas  is  allowed  to  flow 
through  d  until  the  thermometer  in  the  bath  indicates  that  the 
desired  temperature  has  been  reached.  The  flow  of  gas  through 
d  is  then  stopped  by  turning  the  screw  to  the  right.  This  first 
regulation  of  the  temperature  of  the  bath  is  never  exact,  and 
•must  be  followed  by  repeated  slight  readjustments  of  the  mer- 
cury by  means  of  the  screw. 

The  Reichert  regulator  will  enable  one  to  maintain  in  an 
ordinary  bath,  favorably  situated  as  regards  external  conditions, 
a  temperature  which  will  not  vary  more  than  one  or  two  degrees. 
The  sensitiveness  of  the  instrument,  however,  varies  according 
to  the  capacity  of  the  mercury  bulb  and  the  size  and  position 
of  the  outlet  d.  The  hole  through  d  should  be  quite  small  and 
should  be  located  just  above  the  point  where  the  tube  through 
which  the  mercury  rises  begins  to  expand  into  the  larger  chamber 
above.  Many  of  the  instruments  are  of  faulty  construction  in 
these  particulars,  but  a  defective  regulator  may  be  rendered 
serviceable  by  removing  the  glass  stopper  and  replacing  it  by  a 
cork  through  which  is  passed  a  small  glass  tube  drawn  out  to  a 
fine  point.  But  in  this  case  special  provision  must  be  made 
for  the  portion  of  the  gas  supply  which  is  ordinarily  furnished 
through  c  by  inserting  Y-tubes  in  the  rubber  tubes  leading  to 
and  from  the  regulator  and  controlling  the  flow  of  gas  through 
the  branch  by  means  of  a  screw-pinchcock.  Owing  to  the  pres- 
ence of  sulphur  in  the  gas,  the  mercury  and  the  glass  surfaces 


196 


QUANTITATIVE  EXERCISES 


in  the  vicinity  of  d  become  fouled  with  a  black  deposit  which 
diminishes  the  sensitiveness  of  the  instrument.  This  difficulty 
may  be  almost  wholly  overcome  by  passing  the  gas,  before  it 
reaches  the  regulator,  over  the  surface  of  a  concentrated  solu- 
tion of  litharge  in  caustic  soda.  A  good  arrangement  for  the 
purpose  is  a  flask  in  which  the  tube  through  which  the  gas 
enters  or  leaves  is  made  to  end  just  above  the  surface  of  the 
solution,  but  not  quite  in  contact  with  it. 

Another  very  excellent  method  of  securing  constant  temper- 
atures is  by  means  of  baths  heated  by  the  vapors  of  boiling 
liquids.  The  principle  of  this  method  is  often 
employed  in  quantitative  operations,  as,  for 
example,  in  the  Hofmann  and  the  Meyer  meth- 
ods of  determining  molecular  weights,  and  in 
the  heating  of  substances  in  tubes  surrounded 
by  the  vapors  of  boiling  liquids. 

In  the  bath  of  Victor  Meyer,  represented  in 
Fig.  41,  which  is  designed  to  take  the  place 
of  the  ordinary  hot-air  bath,  the  temperature  is 
maintained  by  boiling  a  liquid.  The  quantity 
of  liquid  required  is  very  small,  as  is  also  the 
consumption  of  gas.  The  boiling  of  the  liquid 
is  so  regulated  that  the  vapors  condense  in  the 
lower  half  of  the  glass  tube  a.  The  temperature 
which  will  thus  be  reached  and  maintained  in  the  bath  is  quite 
constant,  but  is  usually  a  few  degrees  below  the  boiling  point 
of  the  liquid.  The  condenser  a  may  be  replaced  by  an  ordinary 
cold-water  condenser,  in  which  case  the  liquid  can  be  boiled 
more  rapidly  in  order  to  secure  a  temperature  in  the  bath  which 
is  only  slightly  below  the  boiling  point  of  the  liquid.  The  ven- 
tilation of  the  bath  is  through  the  tube  b  and  the  small  pivoted 
cover  c.  It  will  be  seen  that  this  arrangement  permits  the 
entrance  into  the  bath  of  the  products  of  combustion  from  the 
burner  beneath.  It  may  therefore  be  necessary,  at  times,  to 
close  b  and  to  provide  for  ventilation  by  inserting  a  cork  in  the 


FIG.  41 


THE  PURIFICATION  OF  SUBSTANCES  197 

cover  through  which  are  passed  two  tubes,  one  reaching  nearly 
to  the  bottom,  while  the  other  ends  near  the  top  of  the  bath. 

The  constancy  of  the  temperature  which  it  is  practicable  to 
maintain  in  a  bath  depends  very  much  upon  external  conditions, 
i.e.  upon  the  fluctuations  in  the  temperature  of  the  air  surround- 
ing it.  It  is  therefore  well,  whenever  practicable,  to  surround 
the  bath  with  nonconducting  materials,  and  to  protect  it  from 
air  draughts  by  means  of  screens,  or  by  placing  it  in  a  wooden 
box  open  on  one  side  only. 

As  a  rule,  metallic  baths  should  be  covered  with  asbestus 
paper  except  at  the  place  where  the  flame  is  applied.  A  per- 
manently adhering  cover  may  be  readily  obtained  by  laying 
the  paper,  thoroughly  wet,  upon  the  metallic  surface  and  manip- 
ulating it  with  the  hands  until  all  of  the  air  beneath  has 
been  driven  to  the  edges  and  expelled. 


Liquid  Baths 

It  is  often  convenient  in  desiccating  substances  and  for  many 
other  purposes  to  employ  water  or  other  liquid  baths  rather 
than  the  ordinary  hot-air  bath.  Under  suitable  conditions  a 
very  satisfactory  constancy  of  temperature  can  be  maintained  in 
such  baths.  A  good  thermo-regulator,  like  the  Reichert,  is  as 
effective  in  a  liquid  as  in  a  hot-air  bath,  and  external  protection 
by  means  of  screens  and  nonconducting  coverings  is  as  advan- 
tageous to  the  former  as  to  the  latter.  In  general,  the  quantity 
of  liquid  in  a  water  bath  which  is  to  be  heated  to  a  temperature 
below  100°  should  be  as  large  as  it  is  practicable  to  make  it, 
and  the  lower  the  temperature  to  be  maintained,  the  larger 
should  be  the  vessel  selected  for  the  bath.  Moreover,  when 
the  temperature  to  be  secured  is  below  the  boiling  point,  the 
water  in  the  bath  requires  constant  agitation.  A  very  effective 
arrangement  for  this  purpose  is  a  stirrer  of  sheet  metal  cut  and 
bent  to  resemble  a  ship's  propeller  and  attached  to  a  vertical 
shaft  which  is  rotated  by  a  small  electric,  hot-air,  t>r  water 


THE 

UNIVERSITY 


198  QUANTITATIVE  EXERCISES 

motor.  In  many  cases  a  sufficient  amount  of  agitation  may  be 
secured  in  the  manner  recommended  by  Ostwald,  who  employs 
an  arrangement  similar  to  a  windmill,  which  is  attached  in  a 
horizontal  position  to  the  upper  end  of  the  shaft  and  kept  in 
motion  by  placing  a  burning  lamp  beneath.  Since  the  amount 
of  power  obtained  in  this  way  is  very  small,  the  agitator  must 
be  constructed  of  light  materials  throughout,  and  pains  must 
be  taken  to  reduce  as  much  as  possible  the  friction  upon  the 
wearing  parts.  The  arms  of  the  windmill  may  be  made  of 
aluminium,  or  of  wire  bent  into  the  proper  form  and  covered 
with  paper.  As  a  means  of  propulsion,  a  small  water  air  blast 
is  more  efficient  than  a  lamp. 

4.  PRECIPITATION 

Some  judgment  must  be  exercised  with  respect  to  the  concen- 
tration of  solutions  from  which  precipitations  are  to  be  made. 
The  fact  that  nearly  all  precipitates,  especially  when  they  are 
first  formed,  are  somewhat  soluble  in  their  mother  liquors  sug- 
gests the  propriety  of  avoiding  unnecessary  dilution  previous 
to  precipitation.  On  the  other  hand,  it  should  not  be  forgotten 
that  a  moderate  degree  of  dilution  is  advantageous  and,  in  many 
cases,  necessary.  Rapidly  forming  precipitates  inclose  portions 
of  the  adjacent  liquid  and  protect  them,  for  a  time  at  least,  from 
the  action  of  the  precipitating  agent.  It  is  obvious  that  the 
incompleteness  of  the  precipitation  due  to  this  cause,  and  also 
the  slowness  with  which  the  precipitation  will  be  completed  in 
consequence  of  diffusion  through  the  protecting  layers  of  pre- 
cipitated material,  must  increase  with  the  concentration  of  the 
solution.  It  is  obvious,  too,  that  precipitates  which  inclose  con- 
centrated mother  liquors  must  be  difficult  to  cleanse  by  subse- 
quent washing  upon  filters. 

It  is  always  necessary  to  add  an  excess  of  the  precipitating 
agent,  and  the  more  soluble  the  precipitate,  the  larger  should 
be  the  excess  of  the  precipitant.  If  a  solution  of  any  silver 


THE  PURIFICATION  OF  SUBSTANCES  199 

salt,  e.g.  silver  nitrate,  is  mingled  with  one  containing  an 
exactly  equivalent  quantity  of  a  chloride,  e.g.  sodium  chloride, 
and  the  resulting  silver  chloride  is  separated  from  the  liquid  by 
nitration,  the  filtrate,  though  perfectly  clear,  will  be  found  to 
contain  both  silver  and  chlorine;  for  if  it  is  divided  into  two 
parts,  and  silver  nitrate  is  added  to  one  and  sodium  chloride  to 
the  other,  both  portions  will  give  precipitates  of  silver  chloride. 
Again,  if  a  solution  of  any  barium  salt,  e.g.  the  chloride,  and 
one  of  any  sulphate,  e.g.  sulphuric  acid,  are  brought  together 
in  equivalent  quantities  and  the  precipitated  barium  sulphate 
is  removed  by  filtration,  the  filtrate  will  give  a  further  precipi- 
tation of  sulphate  when  treated  either  with  barium  chloride  or 
with  sulphuric  acid.  It  is  not  necessary,  in  order  to  obtain  the 
further  precipitation,  to  employ  silver  nitrate  or  sodium  chloride 
in  the  first  case,  and  barium  chloride  or  sulphuric  acid  in  the 
second.  Any  soluble  silver  salt  or  soluble  chloride  in  the  one 
instance,  and  any  soluble  barium  salt  or  soluble  sulphate  in  the 
other,  will  produce  the  same  result.  In  other  words,  it  is  neces- 
sary only  that  the  substance  added  and  the  precipitate  shall 
have  a  constituent  (ion)  in  common.  This  principle  is  of  great 
utility  in  quantitative,  separations  by  precipitation. 

Most  precipitates,  when  first  formed,  are  so  finely  divided 
that  portions  of  them  remain  suspended  for  a  time  in  the  mother 
liquors.  If  immediate  filtration  is  attempted,  the  filters  become 
clogged  and  frequently  cloudy  filtrates  are  obtained.  On  stand- 
ing, in  contact  with  the  mother  liquors,  the  finer  particles  dis- 
appear while  the  larger  ones  grow  to  greater  size  ;  that  is,  the 
precipitates  become  coarser  in  time  and  therefore  much  easier 
to  filter  and  to  cleanse  by  washing.  The  change  in  a  precipi- 
tate from  the  finer  to  the  coarser  condition  is  greatly  facilitated 
by  heat.  These  observations  have  given  rise  to  the  following 
rules :  (1)  Whenever  practicable,  precipitate  from  hot  solutions  ; 
(2)  continue  to  heat  for  some  time  after  precipitation  ;  (3)  never 
attempt  to  filter  until  the  supernatant  liquid  has  become  per- 
fectly clear. 


200  QUANTITATIVE  EXERCISES 

5.  FILTERS 

The  filter  paper  which  is  now  in  common  use  for  quanti- 
tative purposes,  known  as  the  ashless  paper,  consists  almost 
entirely  of  cellulose  and  leaves  a  scarcely  weighable  residue 
when  burned.  It  differs  from  the  paper  formerly  used  in  that, 
in  addition  to  a  treatment  with  hydrochloric  acid  for  the  pur- 
pose of  extracting  inorganic  bases,  it  has  been  subjected  to  the 
action  of  hydrofluoric  acid  for  the  purpose  of  eliminating  silicic 
acid.  These  filters  can  be  employed  with  safety  without  further 
purification  and  the  weight  of  the  ash  may  usually  be  neglected. 
If  other  filter  papers  must  be  employed,  they  should  be  cleansed 
and  the  ash  which  they  leave  when  burned  should  be  deter- 
mined. To  purify  them,  they  are  soaked,  a  large  number  at 
a  time  —  from  24  to  48  hours,  in  quite  dilute  hydrochloric  acid, 
and  then  washed  with  distilled  water  until  the  washings  give 
no  reaction  for  chlorine  with  silver  nitrate.  Owing  to  their 
tenderness  while  wet,  some  care  must  be  exercised  in  manipu- 
lating them,  or  they  will  be  torn  and  more  or  less  disintegrated. 
To  determine  the  ash,  a  number  of  the  papers  (from  5  to  15) 
are  completely  incinerated  in  a  weighed  platinum  crucible  and 
the  weight  of  the  residue  ascertained. 

A  filter  paper  should  be  carefully  fitted  to  the  walls  of  its 
funnel  at  all  points.  This  is  easily  accomplished  if  the  walls 
of  the  funnel  make  the  correct  angle  with  its  axis.  The  paper 
is  folded  symmetrically,  opened  in  the  funnel,  and  formed,  as 
well  as  may  be,  to  the  sides.  It  is  then  wet  with  distilled 
water,  allowed  to  drain,  and  again  fitted  to  the  sides  of  the  fun- 
nel, care  being  taken  not  to  rend  the  paper  by  rough  handling. 
If  the  funnel  is  not  of  correct  form,  the  last  folding  of  the  paper 
is  made  somewhat  unsymmetrically,  and  the  longer  or  shorter 
side  is  opened  according  as  the  angle  of  the  sides  is  too  great  or 
too  small.  The  funnels  selected  for  papers  of  given  sizes  should 
be  only  slightly  larger  than  the  folded  filters,  since  when  the 
edges  of  the  funnels  are  high  above  those  of  the  papers,  it  is 


THE  PURIFICATION  OF  SUBSTANCES  201 

much  more  difficult  to  introduce  without  loss  the  materials  to 
be  filtered  and  the  liquids  which  are  employed  in  washing  the 
precipitates. 

In  filtering  under  diminished  pressure,  the  portion  of  the 
paper  which  extends  into  the  stem  of  the  funnel  requires  sup- 
port. The  platinum  cones  devised  by  Bunsen  are  usually 
employed  for  this  purpose.  They  are  made  in  the  following 
manner:  A  disk  of  platinum  foil  is  cut  in  the 
form  represented  in  Fig.  42,  and  a  mold  for  it 
is  made  by  allowing  plaster  of  Paris  to  harden 
about  the  steel  cone-model  represented  in  Fig.  43. 

The    platinum    disk   is    softened   in  the   flame, 

FIG.  42 
wrapped  about  the  apex  of  the  steel  model,  and 

then  formed  in  the  gypsum  mold.  If  the  platinum  cone  opens 
when  removed  from  the  mold,  it  is  made  a  little  too  small  by 
pressing  the  opposite  sides  with  the  fingers ;  it  is  then  grasped 
with  the  forceps,  reheated,  and  again  formed  in  the  mold.  When 
finished  it  should  perfectly  maintain  its  shape  and  present  no 
visible  opening  at  the  apex.  If  greater  firmness  is  desired,  the 
overlapping  edges  may  be  soldered  at  a  single  point  by  melting 
at  the  proper  place  a  little  borax  and  a  fragment  of 
gold.  Any  change  in  form  due  to  the  last  operation 
is  easily  remedied  by  reshaping  the  cone  in  the 
mold.  Instead  of  a  platinum  cone,  one  made  by 
folding  a  small  disk  of  the  so-called  hardened  paper 
may  be  used  to  support  the  apex  of  the  filter. 

The  Gooch  filter,  which  is  often  used  with  advan- 
FIG.  43 

tage  in  quantitative  work,  requires  a  platinum  cru- 
cible, in  the  bottom  of  which  a  considerable  number  of  minute 
holes  have  been  made.  The  sides  of  the  crucible  are  straight, 
and  the  bottom,  which  is  flat,  is  provided  with  a  closely  fitting 
platinum  cap,  which  is  removed  for  filtering  and  replaced  when 
the  crucible  and  its  contents  are  to  be  heated.  The  filter 
proper  is  made  by  depositing  upon  the  perforated  bottom  of  the 
crucible  a  layer  of  finely  divided  asbestus.  The  depth  of  the 


202  QUANTITATIVE  EXERCISES 

layer  and  also  the  fineness  of  the  asbestus  used  in  making  it 
may  vary  somewhat  according  to  the  coarseness  of  the  material 
to  be  filtered. 

To  prepare  the  asbestus  for  use  in  this  filter,  a  quantity  of 
the  material,  of  good  quality,  is  macerated  with  water  in  a  por- 
celain mortar  until  it  is  reduced  to  a  sufficiently  fine  condition. 
It  is  then  stirred  up  in  a  larger  quantity  of  water  and  the  por- 
tion which  remains  suspended  for  some  time  after  the  water  has 
come  to  rest  is  floated  out.  The  residue  is  soaked  for  several 
hours  in  moderately  strong  hydrochloric  acid,  filtered,  washed 
with  distilled  water,  dried,  and  finally  ignited  in  a 
platinum  dish  over  the  blast  lamp. 
.  In  addition  to  the  perforated  crucible  and  the  pre- 
L  J  pared  asbestus,  there  are  required  for  the  completion 
of  the  Gooch  filter  the  cylindrical  glass  funnel  a, 
Fig.  44,  and  the  rubber  band  b  which  serves  to  hold 
the  crucible  in  place. 

When  a  filter  is  to  be  made,  a  quantity  of  the 
prepared  asbestus  suspended  in  water  is  introduced 
into  the  crucible  (arranged  as  shown  in  the  figure), 
FIG.  44  and  the  water  is  drawn  off  under  moderately  dimin- 
ished pressure.  Other  portions  of  asbestus  are  after- 
wards introduced  and  treated  in  the  same  manner  until  a  filter 
of  sufficient  thickness  has  been  obtained.  Finally,  the  filter  is 
washed  two  or  three  times  with  water  (to  remove  any  material 
which  would  be  likely  to  become  dislodged  and  lost  in  subse- 
quent operations),  dried,  and  heated  to  constant  weight  at  the 
temperature  to  which  the  precipitate  which  it  is  proposed  to 
collect  upon  it  is  to  be  heated.  It  sometimes  happens,  during 
the  collection  and  washing  of  a  precipitate  upon  an  asbestus 
filter,  that  some  of  the  shreds  of  the  filter  are  carried  into  the 
filtrate;  the  latter  should  always,  therefore,  be  examined.  If  it 
is  found  to  contain  fragments  of  asbestus,  these  should  be  col- 
lected upon  a  filter  of  ashless  paper  and  the  loss  to  the  filter 
determined  by  burning  the  paper  and  weighing  the  residue. 


THE   PURIFICATION  OF  SUBSTANCES  203 

An  asbestus  filter  which  is  of  service  in  some  cases  is  pre- 
pared by  placing  a  perforated  porcelain  disk  in  the  bottom  of  a 
cylindrical  funnel,  similar  to  that  represented  in  Fig.  44,  and 
depositing  upon  it  a  layer  of  prepared  asbestus  in  the  same 
manner  as  in  making  the  Gooch  filter. 

The  Monroe  filter,  which  may  often  be  used  with  advantage 
as  a  substitute  for  one  of  asbestus,  is  made  in  the  following 
manner:  A  perforated  Gooch  crucible  is  placed  upon  filter 
or  blotting  paper  and  its  bottom  covered  to  a  depth  of  4  or 
5  mm.  with  moist  ammonium-platinum  chloride.  The  crucible 
is  cleansed  externally,  dried,  covered,  and  placed  in  its  cap. 
The  double  salt  is  then  gradually  decomposed  by  heat.  If 
cracks  appear  in  the  filter,  as  they  are  likely  to  do,  they  are 
filled  with  a  fresh  portion  of  the  double  salt  and  the  crucible  is 
again  heated.  Finally,  the  upper  surface  of  the  filter  of  spongy 
platinum  is  made  smooth  by  gently  rubbing  it  with  the  end  of 
a  glass  rod  which  has  been  rounded  in  the  flame.  All  precipi- 
tates which  can  be  dissolved  in  any  reagent  which  does  not 
attack  platinum  may  be  collected  and  washed  upon  such  a  filter. 
There  is  one  danger,  however,  which  is  not  to  be  overlooked. 
If  the  pores  of  the  filter  are  filled  with  air,  and  a  liquid  containing 
free  hydrochloric  acid  is  passed  through  it,  a  little  platinum  will 
be  dissolved. 

6.  FILTRATION 

Liquids  are  filtered  while  hot,  if  practicable,  because  in  that 
condition  they  pass  through  the  filters  much  more  rapidly  than 
when  cold.  In  quantitative  separations,  however,  it  is  necessary 
to  take  into  account  the  effect  of  temperature  upon  the  solubil- 
ity of  the  material  to  be  collected  on  the  filters. 

In  transferring  a  liquid  and  precipitate  to  a  filter,  it  is  best 
to  introduce  the  greater  portion  of  the  former  as  free  from  the 
latter  as  may  be ;  in  other  words,  it  is  well  not  to  stir  up  the 
precipitate  unnecessarily  until  nearly  all  of  the  liquid  has  been 
passed  through  the  filter.  The  purpose  of  this  is,  of  course,  to 


204  QUANTITATIVE    F.XKIIUSKS 

postpone  as  long  as  possible  the  clogging  of  the  filter  which  is 
sure  to  follow  the  introduction  of  a  finely  divided  precipitate. 
When  only  a  little  of  the  liquid  remains  with  the  precipitate, 
the  latter  is  stirred  up  and  as  much  of  it  as  possible  is  intro- 
duced into  the  filter  with  the  residue  of  the  former.  A  small 
quantity  of  the  filtrate  —  always  less  than  a  filterful  —  is  then 
returned  to  the  vessel  and  more  of  the  precipitate  is  brought 
upon  the  filter  in  the  same  manner.  This  procedure  is  repeated 
until  but  little  of  the  precipitate  remains  except  that  which  is 
attached  to  the  sides  and  bottom  of  the  vessel.  To  complete 
the  transference  of  the  precipitate,  a  small  quantity  of  the  fil- 
trate (3  to  5  cc.)  is  poured  into  the  vessel  and  the  whole  interior 
surface  is  washed  down  with  a  feather  or  with  a  rubber  on  the 
end  of  a  glass  rod.  The  detached  material  is  then  stirred  up 
with  the  liquid  and  brought  upon  the  filter.  This  last  operation 
must  be  repeated  until  no  particles  of  the  precipitate  can  be 
detected  upon  the  sides  or  bottom  of  the  vessel,  and  until  the 
wash  liquid  is  entirely  free  from  any  appearance  of  cloudiness. 

There  are  two  good  reasons  for  the  practice  of  employing  the 
filtrate,  rather  than  water  or  some  other  liquid,  to  bring  the 
precipitate  upon  the  filter.  In  the  first  place,  the  filtrate,  being 
already  saturated,  exerts  no  solvent  action  on  the  precipitate; 
in  the  second  place,  the  volume  of  the  filtrate  is  not  increased 
by  its  use  for  this  purpose  as  it  would  be  if  another  liquid  were 
employed. 

To  prepare  a  feather  for  use  in  detaching  precipitates  from 
the  interior  surfaces  of  vessels,  it  is  trimmed  in  the  following 
manner:  At  a  point  near  the  free  end  —  where  the  rachis  is 
judged  to  be  sufficiently  stiff  and  elastic  —  the  feather  is  cut 
off  transversely  at  an  angle  of  about  30°  to  its  axis.  The  barbs 
on  either  side  are  then  cut  away  for  a  distance  of  about  15  mm. 
in  lines  parallel  to  the  rachis  and  at  a  distance  of  2-4  mm.  from 
it.  The  remainder  of  the  rachis  is  entirely  denuded  of  barbs 
and  made  very  smooth  by  scraping  with  the  edge  of  a  sharp 
knife  blade. 


TIIK    I'UMFICATIOX    OF    SUISTA  NCKS  205 

A  substitute  for  the  feather,  which  is  in  common  use,  is 
familiarly  designated  as  the  "policeman."  It  is  made  from  a 
rectangular  piece  of  sheet  rubber  by  covering  one  side  —  except 
a  narrow  strip  through  the  center  —  with  cement  and  then  once 
folding  the  piece  through  the  middle  at  right  angles  to  the 
uncovered  strip.  A  long  pocket  is  thus  provided,  into  which 
the  end  of  a  glass  rod  may  be  crowded.  This  arrangement, 
though  not  superior  to  the  feather,  is  to  be  preferred  to  a  glass 
rod  with  a  short  piece  of  rubber  tubing  drawn  over  the  end, 
which  is  sometimes  used  hi  cleansing  beakers,  since  the  uncov- 
ered condition  of  the  end  of  the  rod  in  the  latter  affords  the  pre- 
cipitate an  opportunity  to  become  lodged  between  the  rubber  and 
the  glass. 

7.  THE  WASHING  OF  PKECIPITATES 

It  is  important  that  the  washing  of  precipitates  should  be 
accomplished  with  the  least  possible  quantity  of  liquid:  1st, 
because  of  the  solvent  action  of  the  wash  liquid  upon  the  pre- 
cipitate ;  2d,  in  order  to  avoid  an  unnecessary  increase  in  the 
volume  of  the  filtrate;  and  3d,  for  the  purpose  of  saving  time. 
To  this  end  the  filter  pump,  or  some  other  means  of  securing 
diminished  pressure,  should  always  be  employed  in  the  washing 
of  precipitates,  and  before  introducing  a  fresh  portion  of  the 
wash  liquid  as  much  of  the  liquid  already  in  the  filter  should 
be  withdrawn  as  it  is  possible  to  extract  with  its  aid.  How 
irrational  a  different  course  would  be  will  appear  from  the  fol- 
lowing illustration.  We  will  suppose  that  in  washing  a  given 
precipitate  25-cc.  portions  of  water  are  introduced  into  the  filter, 
and  that  after  each  addition  all  but  0.5  cc.  is  withdrawn  with 
the  aid  of  the  puuip.  Theoretically  the  impurities  would  be 
reduced  by  the  first  washing  to  0.02  ;  by  the  second,  to  0.00039 ; 
by  the  third,  to  0.0000077 ;  and  by  the  fourth,  to  0.00000015. 
Suppose,  on  the  other  hand,  that  the  successive  25-cc.  portions 
of  water  are  added  while  5  cc.  of  liquid  remain  in  the  filter. 
Theoretically  the  impurities  will  then  be  reduced  by  the  first 


206  QUANTITATIVE  EXERCISES 

washing  to  0.2;  by  the  second,  to  0.0333;  by  the  third,  to 
0.00556  ;  and  by  the  fourth,  to  0.000926.  A  comparison  of  the 
two  sets  of  figures  will  convince  one  of  the  immense  advantage 
to  be  gained  in  washing  precipitates  by  drawing  off,  as  completely 
as  possible,  the  liquid  already  in  a  filter  before  adding  a  fresh 
portion  of  the  wash  liquid. 

The  above  estimate  of  the  rate  at  which  the  impurities  will 
be  reduced  under  the  given  conditions  is  based  on  the  supposition 
that  when  a  quantity  of  wash  liquid  is  introduced  into  the  filter, 
the  impurities  will  distribute  themselves  uniformly  through  the 
whole  body  of  the  liquid,  and  that  the  portion  which  passes 
through  the  filter  into  the  filtrate  is  therefore  of  the  same  con- 
centration as  that  which  remains  behind  in  immediate  contact 
with  the  precipitate  and  the  material  of  the  filter.  As  a  matter 
of  fact,  however,  the  portion  of  the  solution  which  remains 
behind  and  cannot  be  extracted  by  the  pump  is  always  more 
concentrated  than  the  filtrate.  It  is  for  this  reason  that  the 
cleansing  of  a  precipitate  by  washing  is  found  in  practice  — 
even  under  the  best  management  —  to  be  much  slower  than  is, 
indicated  by  the  computations  just  given.  Nevertheless,  the 
argument  in  favor  of  thoroughly  extracting  the  liquid  already 
in  a  filter  before  introducing  a  fresh  portion  of  the  wash  liquid 
holds  good. 

The  tendency  of  solutions  to  become  more  concentrated  in 
the  immediate  vicinity  of  solids,  e.g.  around  precipitates  and 
the  material  of  filters,  is  known  as  adsorption.  A  familiar  effect 
of  adsorption  is  that  which  is  observed  when  a  standard  solution 
is  passed  through  a  dry  filter;  the  first  portion  of  the 'filtrate  is 
well  known  to  be  weaker  than  the  original  solution,  and  it  is 
therefore  always  rejected. 

Precipitates  which  —  owing  to  their  fine  state  of  subdivision 

—  work  themselves  into  the  interstices  of  the  filter  and  clog  it 

to  such  an  extent  as  seriously  to  interfere  with  filtration  are 

often  partially  washed  by  decantation  before  bringing  them 

upon  the  filter ;  that  is,  they  are  stirred  up  with  portions  of  the 


THE  PURIFICATION  OF   SUBSTANCES  207 

wash  liquid,  which,  when  the  precipitates  have  subsided,  are 
poured  into  the  filter.  This  method  of  cleansing  precipitates  is, 
however,  exceedingly  ineffective,  owing  to  the  large  fraction  of 
the  liquid  which  must  each  time  be  left  with  the  precipitate. 
The  objection  to  it  is  identical  with  that  to  the  practice  of 
introducing  wash  liquid  into  filters  before  removing  as  much  as 
possible  of  the  liquid  already  in  them. 

It  has  been  stated  that  if  a  saturated  solution  of  a  compound 
is  treated  with  a  soluble  substance  containing  one  of  the  con- 
stituents into  which  the  compound  dissociates  in  solution,  some 
of  the  dissolved  material  will  be  precipitated,  and  that,  for  this 
reason,  it  is  customary  to  add  an  excess  of  the  precipitating 
agent.  The  same  principle  may  be  utilized  with  great  advantage 
at  times  to  diminish  the  solubility  of  precipitates  in  the  liquids 
with  which  they  are  washed ;  for  example,  if  a  precipitate  of 
barium  chromate  is  to  be  washed  with  water  containing  a  little 
acetic  acid  or  an  acetate  in  which  it  is  sensibly  soluble,  its  solu- 
tion may  be  almost  entirely  prevented  by  adding  a  little  ammo- 
nium chromate  to  the  wash  liquid.  The  quantity  of  the  salt 
required  for  this  purpose  is  very  small,  and  when  the  other 
impurities  have  been  washed  out  of  the  precipitate  the  little 
ammonium  chromate  introduced  with  the  wash  liquid  may  be 
sufficiently  eliminated  by  a  very  moderate  washing  in  pure  water 
in  which  barium  chromate  is  only  slightly  soluble. 

If  freshly  precipitated  silver  chloride  is  collected  upon  a  filter 
and  is  then  washed  for  a  long  time  with  pure  water  —  especially 
with  cold  water  —  it  will  be  noticed  that  the  filtrate  remains 
perfectly  clear  as  long  as  any  of  the  liquid  in  which  the  precipi- 
tation was  made  is  still  in  the  filter,  and  for  some  time  after 
the  beginning  of  the  washing  with  water ;  that  later,  however, 
the  upper  portions  of  the  filtrate  become  cloudy  from  the  pres- 
ence of  precipitated  silver  chloride.  If  the  receiving  vessel  is 
exchanged  for  an  empty  one,  and  the  washing  with  water  is 
continued,  a  clear  filtrate  will  be  obtained,  which,  nevertheless, 
becomes  cloudy  when  any  acid  or  soluble  salt  is  added  to  it.  On 


208  QUANTITATIVE  EXERCISES 

the  other  hand,  if  a  little  acid  or  a  small  quantity  of  some  salt  is 
added  to  the  water  before  beginning  the  washing  of  the  precipi- 
tate, the  nitrate  will  remain  clear  to  the  end.  The  explanation 
of  this  conduct  is  as  follows:  Silver  chloride,  when  freshly 
precipitated,  belongs  to  a  class  of  bodies  known  as  colloids,  a 
characteristic  of  which  is  the  tendency  to  form  with  pure  water 
so-called  pseudo  solutions,  from  which  they  are  reprecipitated 
by  the  addition  of  acids  or  salts.  The  solution  of  the  silver 
chloride,  therefore,  does  not  begin,  in  the  case  just  cited,  until 
these  substances  have  been  removed  from  the  filter  by  washing, 
and  a  reprecipitation  follows  because  of  their  presence  in  the 
filtrate.  Familiar  examples  of  precipitates  which  behave  in  a 
similar  manner  are  the  hydroxides  of  iron  and  aluminium  and 
the  sulphides  of  the  metals.  Such  substances  are  difficult  to 
cleanse  by  washing,  owing  to  their  voluminous  and  sometimes 
gelatinous  character  and  their  tendency  to  clog  the  filters.  On 
standing  and  when  heated,  they  usually  become  more  compact 
and  therefore  easier  to  wash  upon  filters.  In  the  more  compact 
condition  they  are  also  less  inclined  to  redissolve  —  to  "  run 
through"  the  filter  —  on  continued  washing.  In  dealing  with 
them  it  is  well  to  precipitate  from  hot  solutions  and  to  keep 
the  liquid  hot  for  some  time  after  precipitation ;  to  allow  them 
to  stand  a  long  time  before  attempting  filtration ;  and  to  add  to 
the  wash  water  some  substance  which  will  prevent  a  return  to 
the  pseudo-soluble  condition.  It  appears  to  make  but  little  dif- 
ference what  substance  is  employed  for  the  last  purpose  so  long 
as  it  does  not  affect  the  composition  of  the  precipitate ;  it  is 
therefore  well,  when  permissible,  to  select  ammonium  salts,  since 
these  can  afterwards  be  expelled  from  the  precipitates  by  heat. 


CHAPTER   IX 
SILVER  AND  THE  HALOGENS 

EXERCISE  XVI 
DETERMINATION   OF   CHLORINE   AND   SILVER 

.    I.    GRAVIMETBICALLY  AS  SILVER  CHLORIDE 

Weigh  into  a  beaker  —  preferably  one  having  a  "  lip  "  — 
about  0.2  gram  of  pure  potassium  chloride.  Dissolve  the  salt 
in  75  or  100  cc.  of  water  and  add  slowly,  with  constant  stirring, 
a  dilute  solution  of  silver  nitrate,  to  which  have  been  added 
a  few  drops  of  nitric  acid,  until  it  is  believed  that  the  silver 
nitrate  is  slightly  in  excess.  Let  the  precipitate  subside  and 
then  add  to  the  nearly  clear  supernatant  liquid  a  little  more  of 
the  silver  nitrate.  If  the  cloudiness  does  not  increase,  warm 
the  contents  of  the  beaker  for  half  an  hour  upon  a  sand  bath  or 
a  wire  gauze,  stirring  vigorously  from  time  to  time,  and  then 
set  the  beaker  aside  in  a  dark  place. 

When  the  liquid  above  the  precipitate  has  become  perfectly 
clear,  rub  the  under  side  of  the  beaker  lip  with  a  minute  quan- 
tity of  vaseline  or  with  some  kind  of  grease.  This  is  intended 
to  prevent  the  liquid  from  working  its  way  over  the  edge  and 
running  down  the  outside  of  the  beaker  while  being  transferred 
to  the  filter.  The  precaution,  though  customary,  is  hardly 
necessary,  since  with  proper  manipulation  there  is  no  need  of 
loss  from  this  cause.  Bring  the  precipitate  upon  a  small  ash- 
less  filter  which  is  arranged  for  use  with  the  filter  pump. 

In  transferring  the  contents  of  the  beaker  to  the  filter,  a  glass 
rod  is  held  vertically  with  its  lower  end  reaching  nearly  to  the 

209 


210  QUANTITATIVE  EXERCISES 

bottom  of  the  filter,  and  against  this  is  laid  the  edge  of  the 
beaker  at  the  point  over  which  the  liquid  and  precipitate  are  to 
pass.  In  this  way  the  liquid  is  made  to  glide  quietly  down  the 
rod  and  enter  the  filter  without  spattering.  When  the  filter 
has  been  sufficiently  filled,  the  beaker  is  tilted  backwards,  but 
the  rod  should  not  be  removed  as  long  as  any  liquid  remains 
between  it  and  the  edge.  If  a  little  becomes  lodged  at  the 
point  of  contact,  as  is  usually  the  case,  it  may  be  made  to  flow 
in  the  one  direction  or  the  other  by  changing  the  angle  of  the 
rod  and  by  sliding  it  up  and  down  over  the  edge  of  the  beaker. 
If  the  rod  is  removed  prematurely,  the  liquid  and  precipitate 
which  are  entangled  between  it  and  the  beaker  will  run  over 
the  edge  and  down  the  outside  of  the  latter;  and  when  the  out- 
side has  once  become  wet,  it  is  difficult  to  avoid  a  repetition  of 
the  accident. 

The  precipitate  which  adheres  to  the  beaker  is  brought  into  the 
filter  with  the  aid  of  small  portions  of  the  filtrate  and  a  trimmed 
feather  or  a  "  flag,"  as  described  in  the  preceding  chapter. 

To  wash  the  precipitate,  first  remove  the  mother  liquor  as 
completely  as  possible  with  the  aid  of  the  filter  pump,  and  then, 
after  having  detached  the  pump,  moisten  the  precipitate  and 
paper  with  a  small  quantity  of  cold  water  containing  a  little 
silver  nitrate.  Give  the  impurities  to  be  removed  by  the  wash- 
ing a  little  time  for  diffusion,  and  then,  as  before,  draw  off 
the  liquid  with  the  pump  as  completely  as  possible.  Continue 
washing  the  precipitate  in  this  manner  with  water  containing 
silver  nitrate  until  the  potassium  nitrate  has  been  removed,  and 
then  wash  it  two  or  three  times  in  the  same  way  with  small 
portions  of  cold  water  containing  a  very  little  nitric  acid.  The 
silver  nitrate  is  added  to  the  first  wash  water  to  diminish  the 
solubility  of  the  silver  chloride  and  also  to  prevent  its  return  to 
the  colloidal  condition.  The  nitric  acid  in  the  water  which  is 
employed  in  the  last  stage  of  the  washing  process  does  not 
directly  affect  the  solubility  of  the  chloride,  but  it  effectually 
corrects  the  tendency  to  revert  to  the  colloidal  state. 


SILVER  AND  THE  HALOGENS  211 

Dry  the  filter  at  100°  or  slightly  above  that  temperature. 
Open  it  on  a  piece  of  glazed  paper  and  transfer  the  silver  chlo- 
ride to  a  weighed  porcelain  crucible  which  is  also  placed  upon 
the  paper.  If  necessary,  a  piece  of  stiff  platinum  wire  or  a 
small  platinum  spatula  may  be  employed  to  detach  the  chloride 
which  adheres  to  the  paper,  in  which  case  the  wire  or  spatula 
is  afterwards  to  be  wiped  clean  upon  the  paper.  Fold  the 
paper  in  the  form  of  a  square  by  turning  in  the  edges,  and  roll 
it  together  tightly,  but  in  such  a  way  that  the  half  of  the  paper 
which  has  been  in  contact  with  the  precipitate  is  all  at  one  end. 
Beginning  in  the  middle  of  the  roll,  wind  a  platinum  wire  spi- 
rally about  the  half  which  contains  no  chloride.  The  wire 
should  be  so  long  that  about  75  mm.  of  its  length  will  not  be 
required  in  the  winding.  With  the  unused  end  of  the  wire  in 
one  hand,  hold  the  roll  over  the  crucible  in  a  vertical  position 
with  the  free  end  upward,  and  with  a  lamp  —  burning  with  a 
small  but  good  oxidizing  flame  —  in  the  other  hand  light  it  at 
the  top.  If,  as  the  flame  descends,  portions  of  the  paper  are 
incompletely  burned,  touch  the  parts  which  are  only  charred 
with  the  edge  of  the  flame  as  often  as  may  be  necessary  to  com- 
plete the  incineration.  When  the  roll  has  been  completely 
burned  down  to  the  point  where  the  wire  begins,  turn  it  into  a 
horizontal  position  and  burn  the  part  inclosed  by  the  wire.  It 
is  important  that  no  incompletely  burned  matter  should  drop 
from  the  wire,  since  the  combustion  cannot  be  finished  in  the 
crucible.  The  roll  must  therefore  be  held  very  steadily,  and  to 
this  end  it  is  well  to  support  the  hand  by  resting  it  on  the  table 
or  on  a  block.  If  the  column  of  ash  outside  of  the  wire  curls 
over  and  appears  to  be  in  danger  of  breaking  off  and  falling 
before  the  burning  is  completed,  its  position  should  be  so 
changed  as  to  diminish  the  strain.  Transfer  to  the  crucible 
anything  which  may  have  fallen  upon  the  glazed  paper,  add  a 
few  drops  of  dilute  nitric  acid,  and  evaporate  it  at  a  very  mod- 
erate temperature.  Repeat  the  treatment  with  nitric  acid  and 
the  evaporation,  and  then  add  a  few  drops  of  hydrochloric  acid. 


212  QUANTITATIVE  EXERCISES 

Evaporate  to  dryness  and  heat  the  crucible  until  the  chloride  in  it 
begins  to  fuse  around  the  edges'.  Cool  the  crucible  in  a  desiccator 
and  weigh.  To  remove  the  chloride  from  the  crucible,  introduce 
a  piece  of  zinc  and  a  little  hydrochloric  or  sulphuric  acid. 


Silver  chloride  melts  at  about  450°.  It  is  perceptibly  soluble 
even  in  cold  water,  but  less  so,  as  explained  already,  in  water 
containing  a  small  quantity  either  of  a  soluble  silver  salt  or  of 
a  chloride.  The  presence  of  a  considerable  quantity  of  chlo- 
rides or  of  the  nitrates  of  the  metals  of  the  alkalies  and  alkaline 
earths  increases  its  solubility.  It  is  easily  dissolved  in  solu- 
tions of  potassium  cyanide,  sodium  thiosulphate,  and  ammonia. 

The  method  just  given  suffices  in  the  case  of  most  soluble 
chlorides  for  the  separation  of  chlorine  from  the  metals.  Stan- 
nic, mercuric,  antimony,  platinic,  and  chromic  chlorides  are 
exceptions  to  the  rule.  In  solutions  of  the  first  four,  silver 
nitrate  precipitates,  in  addition  to  the  chlorine,  some  of  the 
metal  with  which  it  was  previously  combined.  In  solutions  of 
chromic  chloride  the  precipitation  is  incomplete.  The  deter- 
mination of  chlorine  in  these  salts  must  be  preceded  by  a  sepa- 
ration of  the  metals.  The  tin  is  best  precipitated  by  ammonium 
nitrate ;  the  antimony  and  mercury,  by  hydrogen  sulphide ;  and 
the  chromium,  by  ammonia.  Solutions  of  platinic  chloride  are 
evaporated  to  dryness  with  sodium  carbonate  and  the  residue  is 
heated  to  the  fusing  point  in  a  platinum  crucible.  The  sul- 
phides of  some  of  the  metals,  when  precipitated  from  solutions 
of  their  chlorides  by  hydrogen  sulphide,  are  inclined  to  carry 
down  with  them  a  portion  of  the  chlorine.  In  such  cases  the 
solution  should  be  dilute  and  fully  saturated  with  the  gas.  It 
should  also  be  allowed  to  stand  for  some  time  after  saturation 
in  a  closed  vessel  before  filtration.  If  practicable,  the  sulphide 
should  be  redissolved  and  again  precipitated. 

Lead,  silver,  and  mercurous  chlorides,  because  of  their  insol- 
ubility, also  require  special  preliminary  treatment.  The  first  is 


SILVER  AND  THE  HALOGENS  213 

decomposed  by  digestion  with  acid  sodium  or  potassium  carbon- 
ate ;  the  second,  by  fusion  with  an  alkaline  carbonate ;  and  the 
third,  by  digestion  with  caustic  soda  or  potash. 


Bromine  and  iodine  are  determined  as  silver  salts  in  the  same 
manner  as  chlorine.  Care  must  be  taken,  however,  not  to  treat 
solutions  of  their  salts  with  nitric  acid  until  after  an  excess  of 
silver  nitrate  has  been  added.  This  precaution  is  especially 
necessary  in  the  case  of  the  iodides,  which,  with  the  exception 
of  silver  iodide,  are  quite  readily  decomposed  by  nitric  acid 
with  liberation  of  iodine. 

Bromine  and  iodine  are  separated  from  the  various  metals  by 
the  same  methods  as  chlorine.  There  are  also  certain  special 
methods  which  may  be  employed  to  separate  iodine  from  some 
of  the  metals. 

To  determine  silver  as  chloride,  the  solution  containing  it  is 
treated,  at  a  temperature  somewhat  below  the  boiling  point, 
with  a  few  drops  of  nitric  acid  and  then  with  a  very  moderate 
excess  of  hydrochloric  acid,  or  of  sodium  or  potassium  chloride. 
The  precipitate  is  washed  on  the  filter,  first  with  cold  water 
containing  a  little  nitric  acid,  and  then  two  or  three  times  with 
pure  water.  In  all  other  respects  the  procedure  is  the  same  as 
in  the  determination  of  chlorine. 


It  is  customary  to  state  the  results  of  quantitative  determi- 
nations in  the  form  of  percentages,  not  of  the  constituents 
determined,  but  of  the  whole  material  or  compound  employed 
in  the  analysis  ;  for  example,  the  result  of  a  determination  of 
chlorine  in  potassium  chloride  would  be  expressed  as  follows : 

Theoretical  percentage  of  Cl  in  KC1  =           47.53 
Found  "  "    "    "     "     =  

Error  =  per  cent 


214  QUANTITATIVE  EXERCISES 

If  the  quality  of  the  work  is  to  be  judged,  it  is  fairer  to  state 
the  results  in  the  form  of  weights,  as  follows : 

Theoretical  weight  of  Cl  in  ....  grm.  KC1  =  .  .  .  .  grm. 
Found  "       "    "    "       "       "       "      =  " 


Error  =  .  .  .  .    " 

It  will  be  seen  that  when  the  results  are  expressed  in  the 
customary  form,  the  effect  of  the  algebraic  sum  of  the  errors  of 
the  analysis  is  inversely  proportional  to  the  quantity  of  the 
material  operated  upon.  In  other  words,  if  large  quantities  of 
material  are  employed,  one  may  work  somewhat  carelessly  and 
nevertheless  obtain  seemingly  good  results.  This  fact  is  of 
course  a  strong  argument  in  favor  of  using  only  small  quanti- 
ties. Another  sufficient  reason  for  the  employment  of  only 
moderate  quantities  of  material  is  to  be  found  in  the  fact  that 
small  precipitates  are  much  more  easily  and  rapidly  cleansed  by 
washing  than  large  ones. 

Much  of  the  time  which  is  usually  spent  in  making  computa- 
tions may  be  saved  by  preparing,  before  beginning  a  piece  of 
quantitative  work,  a  table  of  equivalent  weights  which  includes 
all  of  the  substances  involved  in  the  calculations  to  be  made.  A 
table  of  this  kind  including  nearly  all  the  substances  employed 
in  the  present  chapter  will  be  found  on  the  following  page.  In 
the  horizontal  lines  are  given  the  quantities  of  various  sub- 
stances which  are  equivalent  to  unit  quantities  of  the  substances 
designated  in  the  first  vertical  column.  They  are  found,  of 
course,  by  dividing  the  atomic  or  molecular  weights  —  or  the 
proper  multiples  of  them  —  of  the  substances  whose  symbols 
stand  at  the  head  of  the  several  columns  by  the  atomic  or  molec- 
ular weights  of  those  whose  symbols  are  given  in  the  first 
column.  For  example,  to  find  the  weight  of  bromine  which  is 
equivalent  to  any  unit  weight  of  chlorine,  we  divide  79.35,  the 
atomic  weight  of  the  former,  by  35.18,  the  atomic  weight  of 
the  latter,  and  obtain  2.2555,  which  is  the  number  under  Br 


SILVER  AND  THE  HALOGENS 


215 


$3 


.  o 
So 


00 

* 


cq  to  co  o 
"*  co  co  05 

rH     rH     <M     rH 


•2    =S^^ 
I    SSI2S 


o  «  « 


* 


I 


CO      O      CO 

rH       O      0 


d    i-3    T-J 


CO      O      t»      O      lO 

1C      00      rH      -^      t^ 


00      O      rH       rH 


CO      <M      rH      O 


CO      rH       rH      rH       rH      rH 


rH  rH  00  l^ 

i-l  CO  O  Ci 

CO  O  ^  rH 

Tfl  QO  t~-  O 


O  -^t  O 

(N  O  O 

CO  i— '  O 

i-l  CO  O 


•^ 

o 


3   S 


-^ 

O 


CO     O     CO     O 
O       rH      O      O 


s 


iO  CO  C5 

!>•  CO  O 

O  QO  O 

rH  CO  t— 


rH      rH      rH      O      O 


l>-  O  <N  O5 

oo  ^  co  10 

CO  ^  O  05 

C5  O  rH  1O 


COrHOCOrHrHrHOOOO 


(N     <N     O     O 
•^     O     CO     CO 


(NOOrHOrHOOOOO 


g      O      CO 
iQ 


00      t-;      O      ^      05 

O      <N      rH      rH      O 


O     <N     TP     O5 

S    o    t-    o 

«q    t^    o    ^ 

oo'od 


l^      Tfl 

^O     *O 


CCr1rHCOrHrHrHOOOO(MCO<N 


CO      O      00      CO      T*      !>•      t- 
O      "^      O       G<1      rH      rH      t— 

co    "*    ^    i>-    t>-    oo    0 


OrHOOOOOrHC^rH 


CO  "<* 

CO  O5 

•«*  t^ 

Tti  (M 


rHOOOOOOOOOOOrHO 


— •     ^  O    P3    i— i 

O     PQ     h-t      bC     bD     bO 


r   A         ^bcOPQhHbcM'bJD^^^ 


216  QUANTITATIVE  EXERCISES 

and  opposite  Cl  in  the  table.  Again,  if  K2O2O7  acts  upon  an 
iodide  in  an  acid  solution,  there  are  liberated  six  atoms  of  iodine 
for  each  molecule  of  the  bichromate,  and  to  find  the  weight  of 
the  former,  which,  in  such  a  reaction,  is  equivalent  to  a  unit 
weight  of  the  latter,  we  divide  six  times  the  atomic  weight  of 
iodine  (6  x  125.89)  by  292,  the  molecular  weight  of  the  bichro- 
mate. The  result  is  2.5867,  which  is  the  number  in  the  table 
under  I  and  opposite  K2Cr2O7.  The  manner  of  using  such  a 
table  is  obvious,  but  its  usefulness  will  be  illustrated  by  a  few 
examples.  Suppose  a  quantity  of  potassium  chloride,  IF,  has 
been  weighed  out,  and  it  is  desired  to  know  what  weight  of 
silver  chloride  it  should  yield,  we  find  in  the  table  opposite 
KC1  and  under  AgCl  the  number  1.9228 ;  the  weight  of  the 
silver  salt  is  W  x  1.9228.  Suppose  a  weight  of  silver  chloride, 
Wf,  has  been  obtained,  and  we  desire  to  find  the  weight  of  the 
chlorine  in  it ;  under  Cl  and  opposite  AgCl  is  given  the  equiva- 
lent 0.2472,  and  the  weight  of  the  chlorine  is  W,  x  0.2472. 
Suppose  again  that  we  have  a  given  weight  of  arsenious  oxide, 
Wn,  and  are  required  to  ascertain  how  much  potassium  bichro- 
mate will  be  needed  for  its  oxidation  to  arsenic  acid ;  by  refer- 
ring to  the  table  we  find  the  weight  to  be  Wn  x  0.9904. 

II.     VOLUMETBICALLY    BY'  MOHR's    METHOD 

One-tenth  normal  solutions  of  potassium  chloride  and  of  silver 
nitrate  are  required. 

To  prepare  the  former,  weigh  out  about  7.5  grams  of  pure 
potassium  chloride.  The  quantity  must  not  be  less  than  7.4, 
and  should  not  exceed  7.6  grams.  Dissolve  the  salt  and  dilute 
the  solution  in  a  liter  measuring  flask  to  the  mark  on  the  neck. 
Divide  the  weight  of  the  salt  in  solution  by  the  capacity  of  the 
flask  in  cubic  centimeters  to  find  the  amount  of  the  salt  in  one 
cubic  centimeter,  and  then  divide  7.4,  the  weight  of  the  salt  in 
one  liter  of  a  tenth-normal  solution,  by  the  quotient.  The 
second  quotient  will  be  the  number  of  cubic  centimeters  of  the 


SILVER  AND  THE   HALOGENS  217 

solution  which  must  be  diluted  to  one  liter,  and  the  difference 
between  this  and  1000  will  be  the  volume  of  the  water  required 
for  the  dilution.  Remove  from  the  flask  with  a  pipette  a  quan- 
tity of  the  solution  which  is  somewhat  greater  than  that  of  the 
water  to  be  introduced.  Measure  in  the  required  volume  of 
water  and  fill  the  flask  to  the  mark  with  the  solution  which  was 
withdrawn.  Close  the  flask  and  thoroughly  mix  its  contents. 
The  solution  should  have,  at  the  time  of  its  dilution,  very 
nearly  the  standard  temperature. 

To  prepare  the  solution  of  the  silver  salt,  dissolve  about  21 
grams  of  neutral  silver  nitrate  in  1200  cc.  of  water.  Fill  one 
burette  with  the  tenth-normal  potassium  chloride  solution,  and 
another  with  the  solution  of  silver  nitrate.  Measure  into  a 
beaker  10  cc.  of  the  chloride  solution.  Add  about  50  cc.  of 
water  and  a  few  drops  of  a  solution  of  neutral  potassium  chro- 
mate  which  is  free  from  chlorine.  The  liquid  in  the  beaker, 
after  the  addition  of  the  chromate,  should  have  a  distinct  but 
not  a  deep  yellow  color.  Titrate  the  solution  of  the  chloride, 
stirring  it  constantly,  with  the  solution  of  silver  nitrate  until  a 
slight  but  permanent  red  color  is  obtained.  Repeat  the  experi- 
ment several  times  with  fresh  portions  of  the  chloride.  Having 
found  the  number  of  cubic  centimeters  of  the  silver  solution 
which  are  equivalent  to  ten  of  the  chloride,  multiply  by  100. 
The  product  will  be  the  number  which  must  be  diluted  to  a 
liter.  Measure  the  water  required  for  the  dilution  into  a  dry 
liter  flask,  and  fill  to  the  mark  with  the  solution  of  silver 
nitrate.  The  two  solutions  should  be  equivalent;  determine 
whether  they  are  so. 

Weigh  off  from  0.1  to  0.2  gram  of  pure  potassium  chloride, 
and  determine  the  chlorine  in  it  by  means  of  the  standard 
solution  of  silver  nitrate. 

Barium  and  lead  form  nearly  insoluble  chromates  ;  therefore  to 
determine  chlorine  in  combination  with  these  metals  it  is  neces- 
sary to  add  a  slight  excess  of  the  chromate,  —  i.e.  somewhat 
more  than  is  required  for  their  precipitation.  The  titration 


218  QUANTITATIVE  EXERCISES 

may  then  be  made  without  removal  of  the  insoluble  chromates. 
Or  the  metals  may  be  precipitated  as  sulphates  with  sodium 
sulphate. 

Silver  chromate  does  not  form  in  the  presence  of  acids,  and 
silver  oxide  is  precipitated  by  the  alkalies;  hence  all  the  solu- 
tions employed  in  connection  with  the  method  of  Mohr  must 
be  neutral,  and  the  presence  of  reducing  substances  is,  of  course, 
inadmissible. 

Silver  chromate  is  soluble  in  6666  parts  of  water  at  17.5°, 
and  in  3704  parts  at  100°  ;  if,  therefore,  potassium  chromate  is 
added  to  a  solution  of  silver  nitrate,  a  red  precipitate  is  obtained, 
which  disappears  rather  slowly  on  titrating  with  a  solution  of  a 
chloride.  For  this  reason  it  is  not  advisable,  when  silver  is  to 
be  determined,  to  treat  its  solution  with  the  chromate  and  then 
to  titrate  with  a  standard  solution  of  some  chloride  until  the 
red  color  is  destroyed.  It  is  much  better  to  add  at  once  an 
excess  of  the  chloride,  and  then  to  determine  the  amount  of  the 
excess  by  means  of  a  standard  solution  of  silver  nitrate. 

Bromine  and  iodine  may  also  be  determined  by  the  method 
of  Mohr. 

III.     VOLUMETRICALLY    BY    VOLHARDJS    METHOD 

Dissolve  about  10  grams  of  ammonium  sulphocyanate  in 
1200  cc.  of  water.  Fill  one  burette  with  the  solution,  and 
another  with  the  tenth-normal  solution  of  silver  nitrate.  Meas- 
ure 10  cc.  of  the  latter  into  a  beaker.  Add  5  cc.  of  a  cold  satu- 
rated solution  of  ferric  ammonium  sulphate  and  about  50  cc.  of 
water.  Stir  in  dilute  nitric  acid,  drop  by  drop,  until  the  solution 
becomes  nearly  colorless.  Titrate  with  the  solution  of  sulpho- 
cyanate until  a  faint  but  permanent  red  color  appears.  Repeat 
the  experiment  several  times.  Having  found  the  relation  of 
the  two  solutions,  dilute  that  of  the  sulphocyanate  to  the  tenth- 
normal  standard,  proceeding  as  directed  under  II. 

If  the  ammonium  sulphocyanate  is  quite  wet  at  the  time  of 
weighing,  the  preliminary  solution  of  it  may  be  found  to  be  top 


SILVER  AND  THE  HALOGENS  219 

weak ;  it  is  therefore  well,  in  such  a  case,  to  weigh  out  some- 
what more  than  the  prescribed  quantity  of  the  salt.  But  if 
the  required  additional  quantity  has  been  considerably  overesti- 
mated, giving  a  much  too  concentrated  solution  of  the  sulpho- 
cyanate,  it  should  be  diluted  before  making  the  determinations 
on  which  the  final  dilution  to  the  standard  is  to  be  based.  As 
a  rule,  solutions  which  are  to  be  diluted  to  any  fixed  standard 
should  have,  at  the  time  of  determining  their  strength,  very 
nearly  their  final  concentration. 

The  nitric  acid  (specific  gravity  1.2)  which  is  used  to  decol- 
orize the  indicator  should  be  boiled  to  free  it  from  nitrous  acid 
and  oxides  of  nitrogen. 

Dissolve  a  weighed  silver  ten-cent  piece  in  dilute  nitric  acid 
and  evaporate  nearly  all  the  excess  of  the  acid.  Dilute  with 
water  and  boil  the  solution  to  remove  nitrous  acid  and  oxides 
of  nitrogen.  Pour  the  solution,  when  cold,  into  a  100-cc.  meas- 
uring flask,  add  the  washings,  and  dilute  to  the  mark  with  water. 
Determine  the  silver  in  measured  portions  of  the  solution. 

According  to  Volhard,  silver  may  be  determined  in  the  pres- 
ence of  copper  without  difficulty  provided  the  ratio  of  the  latter 
to  the  former  does  not  exceed  that  of  7  to  10.  In  the  presence 
of  nickel  and  cobalt  the  determination  is  satisfactory,  but  requires 
some  practice.  Mercury  and  palladium,  on  the  other  hand,  must 
be  removed  before  the  silver  can  be  estimated. 

Weigh  out  from  0.1  to  0.2  gram  of  pure  potassium  chloride 
and  dissolve  it  in  about  100  cc.  of  water.  Add  5  cc.  of  the 
iron  solution  and  decolorize  with  nitric  acid.  Measure  in  an 
excess  of  the  standard  solution  of  silver  nitrate  —  about  twice 
the  quantity  required  to  precipitate  the  chlorine  —  and  then 
titrate  with  the  solution  of  sulphocyanate,  stirring  constantly, 
until  the  liquid  assumes  a  light  yellowish-brown  color.  The 
difference  between  the  quantity  of  silver  added  in  the  first 
place  and  that  found  by  the  subsequent  titration  with  the  sul- 
phocyanate is  the  quantity  which  was  required  to  precipitate 
the  chlorine. 


220  QUANTITATIVE  EXERCISES 

The  end-reaction  when  chlorine  is  determined  in  the  manner 
prescribed  above  is  much  less  striking  than  when  silver  is  pre- 
cipitated by  sulphocyanate  in  the  absence  of  silver  chloride,  and 
the  color  due  to  the  formation  of  ferric  sulphocyanate  is  not  per- 
manent if  the  excess  of  sulphocyanate  is  small.  This  is  due  to 
a  reaction  between  the  coloring  substance  and  silver  chloride  : 

Fe(CNS)3  +  3  AgCl  =  3  AgCNS  +  FeCl3. 

A  little  experience,  however,  enables  one  to  determine  with  ease 
and  certainty  when  the  precipitation  of  the  silver  is  complete. 
It  is  well,  when  this  method  of  determining  chlorine  is  employed 
for  the  first  time,  to  continue  adding  the  sulphocyanate  until  a 
deep  color  is  developed,  and  then  to  add  silver  nitrate  until  it 
is  destroyed.  A  few  such  titrations,  back  and  forth,  in  the 
same  solution,  with  close  observation  of  the  changes  in  color, 
will  prepare  one  sufficiently  for  accurate  work.  The  difficulty 
resulting  from  the  reaction  between  the  ferric  sulphocyanate 
and  the  silver  chloride  may  be  avoided  in  the  following  man- 
ner: The  solution  of  the  chloride,  the  indicator,  and  the  excess 
of  silver  nitrate  are  placed  in  a  small  measuring  flask  and 
well  shaken.  The  flask  is  filled  to  the  mark  with  water,  the 
contents  thoroughly  mixed,  and  then  quickly  filtered  through 
a  dry  paper,  the  first  portions  of  the  filtrate  being  discarded. 
The  excess  of  silver  is  then  determined  by  titrating  meas- 
ured portions  of  the  filtrate  with  the  standard  solution  of 
sulphocyanate. 

When  the  quantity  of  .chlorine  to  be  determined  is  not  even 
approximately  known,  it  is  recommended  to  add  a  drop  or  two 
of  the  sulphocyanate  solution  from  time  to  time  while  measur- 
ing in  the  silver  nitrate.  As  long  as  any  chlorine  remains 
unprecipitated,  the  color  produced  by  the  sulphocyanate  will 
disappear  slowly,  but  when  the  silver  is  in  excess,  it  disappears 
instantly.  The  quantity  of  sulphocyanate  introduced  in  this 
way  must,  of  course,  be  added  to  that  which  is  subsequently 
used  in  determining  the  excess  of  silver. 


SILVER  AND  THE  HALOGENS  221 

When  the  silver  is  standardized  in  the  manner  prescribed 
under  II,  and  afterwards  used  for  the  determination  of  chlorine 
by  the  method  of  Volhard,  the  results  are,  as  a  rule,  somewhat 
low  in  consequence  of  the  reaction  between  the  ferric  sulpho- 
cyanate  and  the  silver  chloride.  It  is  therefore  recommended 
that  a  solution  of  silver  nitrate  which  is  to  be  used  principally 
for  the  estimation  of  chlorine  by  this  method  be  especially  stand- 
ardized for  chlorine.  For  this  purpose  a  solution  of  a  weighed 
quantity  of  a  pure  chloride  is  treated  with  the  indicator  and  an 
excess  of  the  preliminary  solution  of  silver  nitrate,  and  then 
titrated  with  any  very  dilute  solution  of  ammonium  sulphocy- 
anate.  Having  found  in  this  way  the  volume  of  the  sulpho- 
cyanate  solution  required  to  precipitate  the  excess,  the  relation 
of  the  silver  and  sulphocyanate  solutions  to  each  other  is  ascer- 
tained. The  difference  between  the  volume  of  the  silver  nitrate 
added  to  the  chloride  and  that  found  to  be  equivalent  to  the 
sulphocyanate  which  was  used  in  precipitating  the  excess  of 
silver  is  the  volume  of  the  silver  nitrate  which  is  equivalent  to 
the  weighed  quantity  of  chloride.  A  solution  which  is  stand- 
ardized in  this  manner  will  give  correct  results  when  employed 
for  the  estimation  of  chlorine  by  the  method  of  Volhard,  but  it 
must  be  restandardized  as  directed  under  II  when  the  method  of 
Mohr  is  to  be  used. 

Bromine  is  determined  by  the  method  of  Volhard  in  precisely 
the  same  manner  as  chlorine.  The  end-reaction,  however,  is 
more  distinct  and  permanent,  owing  to  the  fact  that  silver  bro- 
mide is  decomposed  by  ferric  sulphocyanate  much  more  slowly 
than  silver  chloride. 

To  determine  iodine,  the  iodide  dissolved  in  two  or  three 
hundred  times  its  weight  of  water  is  placed  in  a  bottle  having 
a  ground-glass  stopper.  The  silver  nitrate  is  added  in  small 
quantities,  and  after  each  addition  the  bottle  is  closed  and  the 
contents  are  well  shaken.  This  treatment  is  continued  until 


222  QUANTITATIVE  EXERCISES 

the  silver  iodide  separates  and  leaves  the  supernatant  liquid 
quite  clear.  A  little  more  silver  nitrate,  also  the  indicator, 
and  the  nitric  acid  necessary  to  bleach  it  are  then  added,  and 
the  excess  of  the  silver  is  determined  with  the  sulphocyanate. 
But  —  since  silver  iodide  carries  down  with  it  a  quantity  of 
silver  nitrate,  and  the  color  indicating  the  end  of  the  reaction  is 
not  permanent  until  the  nitrate  thus  precipitated  has  all  been 
converted  into  sulphocyanate  —  the  standard  solution  of  sulpho- 
cyanate must  be  added  in  small  quantities  and  the  contents  of 
the  bottle  must  be  vigorously  agitated  after  each  addition. 


The  method  of  Volhard  is  usually  preferred  to  that  of  Mohr 
for  the  reason  that  by  it  silver  and  the  halogens  may  be  deter- 
mined in  the  presence  of  nitric  acid  and  therefore  in  solutions 
containing  substances  which,  in  the  absence  of  nitric  acid, 
reduce  the  salts  of  silver.* 


EXERCISE  XVII 
IODOMETRIC   DETERMINATIONS 

These  are  all  based  on  the  reaction  which  takes  place  between 
free  iodine  and  sodium  thiosulphate : 

2  Na2S2O3  +  2  I  -  2  Nal  +  Na2S4O6. 

It  will  be  seen  that  by  the  aid  of  this  reaction  it  is  practicable 
to  determine  all  of  those  substances  which  liberate  from  iodides 
a  definite  quantity  of  iodine,  and  all  of  those  also  which  convert 
into  iodides,  or  absorb,  a  definite  amount  of  free  iodine.  The 
following  equations  represent  the  reactions  with  iodine  or  the 

*  The  student  should  also  familiarize  himself  with  the  volumetric  methods 
of  Gay-Lussac,  of  Pisani,  and  of  Bohlig.  See  Fresenius,  Quant.  Analyse,  1, 
302,  309,  472. 


SILVER  AND   THE   HALOGENS  223 

compounds  of  iodine,  of  some  of  the  substances  which  are  deter- 
mined by  the  iodometric  method  : 

C12  +  2  KI  =  2  KC1  +  I2, 

Br2  +  2  KI  =  2  KBr  +  I2, 

HC10  +  2  HI  =  H20  +  HC1  +  Ia, 

HBrO  +  2  HI  =  H2O  +  HBr  +  I2, 

HC1O3  +  6  HI  =  3  H2O  -f  HC1  +  3  I2, 

HBr03  +  6  HI  =  3  H2O  +  HBr  +  3  I2, 

HIO3  +  5  HI  -  3  H2O  +  3  I2, 


HaS08  -f  H20  +  Ia  =  H2S04  +  2  HI, 
As2O3  +  2  H2O  +  2  I2  =  As2O5  -f  4  HI, 
Sb203  +  2  H20  +  2  I2  =  Sb205  +  4  HI, 
2  CrO3  +  6  HI  =  Cr2O3  -f  3  H2O  +  3  Ia, 
Mn02  +  2  HI  =  MnO  +  H2O  +  Ia. 


I.   PREPARATION  OF  STANDARD  SOLUTIONS 
a.   Resublimed  Iodine 

Select  two  crystallizing  dishes  of  the  same  diameter  whose 
edges  fit  together  quite  closely.  Place  in  one  of  them  a  mix- 
ture of  10  or  15  grams  of  commercial  iodine  and  2  or  3  grams 
of  pulverized  potassium  iodide.  Press  the  dish  halfway  through 
a  hole  which  has  been  made  for  it  in  a  piece  of  asbestus  board 
150  or  200  mm.  square,  and  bring  under  it  a  sand  or  asbestus 
bath  which  is  narrower  than  the  board.  Heat  the  iodine  slowly, 
allowing  the  vapors  to  escape  until  it  is  believed  that  all  the 
water  has  been  expelled,  and  then  invert  over  it  the  other  crys- 
tallizing dish.  Distill  the  iodine  very  slowly,  and  keep  the 
upper  dish  cool  by  placing  upon  it  a  flat-bottomed  metallic  dish 
containing  cold  water.  When  the  sublimation  is  finished, 
transfer  the  iodine  to  a  glass-stoppered  weighing  flask  and  place 
the  latter  in  a  desiccator.  The  purified  iodine  is  used  in  stand- 
ardizing thiosulphate  solutions. 


224  QUANTITATIVE  EXERCISES 

5.    Potassium  Iodide 

Aqueous  solutions  of  potassium  iodide  are  employed  as  a  sol- 
vent for  free  iodine  and  also  as  the  source  of  the  iodine  which 
is  liberated  in  various  reactions  utilized  in  iodometric  processes. 

The  salt  must  be  tested  for  the  presence  of  iodate.  For  this 
purpose  a  small  quantity  of  the  iodide  is  dissolved  in  recently 
boiled  water  and  treated  with  a  little  well-boiled  dilute  hydro- 
chloric acid.  If  the  solution  remains  colorless  for  a  few  min- 
utes, it  is  free  from  iodates.  Starch  paste  may  be  added  to 
facilitate  the  detection  of  free  iodine.  Within  a  short  time  the 
solution  will  become  colored  in  consequence  of  the  action  of 
the  free  oxygen  in  it  on  the  hydriodic  acid  which  is  liberated 
by  the  hydrochloric  acid.  If  the  iodide  is  found  to  contain 
iodate,  the  latter  may  be  removed  by  boiling  the  water  solution 
of  the  salt  with  zinc  amalgam  ,until  it  no  longer  gives  the  reac- 
tion for  iodic  acid  when  treated  with  hydrochloric  acid  and  starch 
paste.  Neither  zinc  nor  mercury  is  dissolved  when  a  solution 
of  an  iodide  is  treated  in  this  manner.  The  former  is  converted 
into  hydroxide,  which  may  be  removed  by  filtering  through  a 
paper  which  has  been  thoroughly  wet  with  boiling  water. 

c.    Starch 

Grind  about  one  gram  of  arrowroot  starch  with  a  little  water 
and  pour  the  mixture  into  150  or  200  cc.  of  boiling  water. 
Continue  to  boil  the  liquid  for  about  one  minute,  and  then 
allow  it  to  cool  and  settle.  The  clear  portion  only  is  to  be 
used  for  the  detection  of  free  iodine.  A  new  preparation  of 
starch  is  to  be  made  for  each  day's  work. 

d.    The  Solution  of  Iodine 

To  prepare  this,  measure  into  a  liter  flask  a  volume  of  potas- 
sium iodide  solution  which  contains  18  grams  of  the  salt. 
Weigh  in  a  little  over  13  grams  of  commercial  iodine.  Close 


SILVER  AND  THE  HALOGENS  225 

the  flask  and  shake  it  until  the  iodine  is  dissolved.  If  the  solu- 
tion progresses  too  slowly,  it  may  be  hastened  by  adding  more 
of  the  potassium  iodide.  Introduce  200  or  300  cc.  of  water 
and  agitate  again.  Continue  to  add  water  and  to  agitate  .after 
each  addition  until  the  flask  is  filled  to  the  mark,  making  sure 
that  all  of  the  iodine  is  dissolved. 


e.    The  Solution  of  Sodium  Thio sulphate 

Weigh  about  27.5  grams  of  the  salt  into  a  liter  flask.     Dis- 
solve it  in  water  and  dilute  to  the  mark. 


/.    Standardization  of  Solutions  d  and  e 

Fill  one  burette  with  the  solution  of  thiosulphate  and  another 
with  that  of  iodine.  Measure  10  cc.  of  the  former  into  a  beaker. 
Dilute  with  water  and  add  1  or  2  cc.  of  tho  starch  preparation. 
Titrate  with  the  iodine  solution,  stirring  constantly,  until  a  faint 
blue  color  is  developed  which  disappears  on  the  addition  of  a 
very  minute  quantity  of  thiosulphate.  Repeat  the  experiment 
until  constant  results  are  obtained. 

Having  found  the  relation  of  the  iodine  and  thiosulphate 
solutions,  it  remains  to  determine  the  exact  strength  of  the 
latter.  To  do  this,  measure  about  10  cc.  of  a  potassium  iodide 
solution  (10  :  1)  into  a  glass-stoppered  flask,  and  weigh  in  from 
0.1  to  0.2  gram  of  the  resublimed  iodine.  Close  the  flask  and 
shake  it  until  the  iodine  is  dissolved.  Add  a  little  water,  then 
close  the  flask  and  shake  it  again.  Proceed  in  this  way  until  a 
solution  is  obtained  which  is  so  dilute  that  no  vapors  of  iodine 
can  be  seen  above  it.  Add  now  the  thiosulphate  until  the 
color  of  the  solution  has  nearly  disappeared,  and  then  1  or  2  cc. 
of  the  starch.  Add  thiosulphate  again  until  the  solution 
becomes  quite  colorless,  noting  carefully,  however,  the  quantity 
required  just  to  destroy  the  blue  color.  From  another  burette 
add  the  iodine  solution  d  until  a  faint  blue  color  appears. 


226  QUANTITATIVE  EXERCISES 

Subtract  from  the  total  volume  of  the  thiosulphate  used  the 
quantity  equivalent  to  the  iodine  solution  d  which  was  used  to 
restore  the  blue  color.  The  difference  is  the  volume  of  the 
thiosulphate  which  is  equivalent  to  the  known  weight  of  pure 
iodine.  Repeat  the  experiment  with  another  weighed  portion 
of  iodine.  Having  found  the  strength  of  the  thiosulphate  solu- 
tion, and  indirectly  that  of  the  iodine,  the  two  solutions  may  be 
diluted  to  any  desired  standard.  One-tenth  normal  solutions 
are  usually  employed,  though  it  is  often  advantageous  to  dilute 
to  one-half  and  even  to  one-tenth  of  that  strength. 

Neither  solution  is  stable,  and  both  require  frequent  restand- 
ardization.  They  lose  strength  less  rapidly  when  kept  in  a 
cool,  dark  place. 

It  has  been  proposed  to  employ  for  the  restandardization  of 
the  thiosulphate  standard  solutions  or  weighed  quantities  of 
stable  compounds  which,  when  acidified,  in  the  presence  of  potas- 
sium iodide,  liberate  a  definite  quantity  of  iodine.  The  follow- 
ing equations  represent  some  of  the  reactions  which  have  been 
utilized  in  this  way: 

K2Cr2O7  + 14  HC1  +  6  KI  =  8  KC1  +  2  CrCl3  +  7  H2O  +  3 12, 
2  KMnO4  +16  HC1  +  10  KI  =12  KC1+  2  MnCl2+  8  H2O  +  5I2, 
KI03  +  6  HC1  +  5  KI  =  6  KC1  +  3  H2O  +  3  I2, 
HIO3.KIO3  +  11  HC1  +  10  KI  =  11  KC1  +  6  H2O  +  6  I2, 
NaBrO3  +  6  HC1  +  6  KI  =  6  KC1  +  NaBr  +  3  H2O  +  3 12. 

II.    IODOMETRIC  DETERMINATION  OF  SULPHUROUS  ACID 

Weigh  about  5  grams  of  sodium  sulphite  into  a  half-liter 
measuring  flask.  Dissolve  the  salt  in  water  and  dilute  to  the 
mark.  Measure  10  cc.  of  the  standard  solution  of  iodine  into  a 
beaker  and  dilute  with  water.  Add  1  or  2  cc.  of  dilute  hydro- 
chloric acid,  and  then  titrate  with  the  sulphite  solution  until 
the  solution  of  iodine  has  become  nearly  colorless.  Add  starch 
and  then  more  of  the  sulphite  until  the  blue  color  has  wholly 
disappeared.  Titrate  back  to  color  with  the  standard  solution 


SILVER  AND  THE  HALOGENS  227 

of  iodine.  The  volume  of  the  iodine  solution  employed  to 
restore  color  must,  of  course,  be  added  to  the  10  cc.  which  were 
measured  out  in  the  first  place.  Calculate  from  the  results  the 
percentage  of  Na2SO3  in  the  sulphite. 

Repeat  the  experiment,  omitting  the  hydrochloric  acid. 

The  order  of  the  titration,  i.e.  the  addition  of  the  sulphite  to  the 
iodine,  cannot  be  reversed  except  in  the  case  of  exceedingly  dilute 
solutions,  because  in  even  moderately  dilute  solutions  of  sulphu- 
rous acid  the  hydriodic  acid  which  is  formed  when  iodine  is  intro- 
duced reacts  upon  a  portion  of  the  remaining  sulphurous  acid 
with  formation  of  water  and  liberation  of  sulphur  and  iodine : 

H2S03  +  4  HI  =  3  H20  +  S  +  2  I2. 

The  iodine  thus  liberated  reacts  at  once  upon  another  portion  of 
the  sulphurous  acid  and  water  with  formation  of  sulphuric  acid 
and  more  hydriodic  acid.  Hence  the  reaction  between  sulphu- 
rous acid  and  iodine  is  regular,  i.e.  in  accordance  with  the  equa- 
tion H2SO3  +  H2O  +  I2  =  H2SO4  +  2  HI,  only  when  the  iodine 
is  in  excess. 

The  reaction  Na2SO3  +  H2O  +  I2  =  Na2SO4  +  2  HI  is  some- 
times employed  in  standardizing  acids.  If  the  strength  of  the 
iodine  solution  is  known,  and  the  sulphite  is  neutral,  the  quan- 
tity of  hydriodic  acid  which  will  be  formed  can  be  calculated. 
It  may  be  employed  to  standardize  a  solution  of  iodine.  For 
this  purpose  a  solution  of  neutral  sulphite  of  unknown  strength, 
but  dilute,  is  added  to  a  measured  portion  of  the  iodine  solution 
to  be  standardized  until  the  color  disappears.  The  hydriodic 
acid  is  then  determined  by  means  of  some  standard  alkali. 
Methyl  orange  should  be  used  as  the  indicator. 

It  will  be  seen  from  the  following  equations  that  it  is  practicable, 
with  a  standard  solution  of  iodine,  to  determine  a  neutral  sulphite 
and  a  thiosulphate  when  the  two  occur  in  the  same  solution : 

2  Na2S203  + 12  =  2  Nal  +  Na2S4O6, 
Na2S03  +  H20  +  Ia  =  Na2S04  +  2  HI. 


228  QUANTITATIVE  EXERCISES 

The  sum  of  the  sulphite  and  thiosulphate  is  found  from  the 
iodine  consumed  in  both  reactions,  while  the  sulphite  is  esti- 
mated separately  by  neutralizing  the  hydriodic  acid  with  a 
standard  alkali. 


III.    IODOMETRIC  DETERMINATION  OF  CHROMIC  ACID 

Weigh  from  0.1  to  0.2  gram  of  pure  potassium  bichromate 
into  a  chlorine  apparatus  of  the  form  represented  in  Fig.  45. 
Drop  in  a  small  piece  of  compact  magnesite  which  is  free  from 

ferrous  iron,  and  fill  the  flask  half  full 
of  concentrated  hydrochloric  acid  which 
contains  no  free  chlorine.  Pour  about 
50  cc.  of  potassium  iodide  solution  of 
FlG-  45  the  usual  concentration  (1:10)  into  the 

receiver  and  connect  the  two  parts  of  the  apparatus  as  shown  in 
the  figure.  Cautiously  heat  the  flask  until  the  reduction  of  the 
chromate  is  complete  and  all  of  the  chlorine  has  been  driven 
over  into  the  receiver.  With  the  lamp  in  one  hand  continue  to 
heat  the  flask,  and  with  the  other  hand  withdraw  the  receiver 
until  the  curved  end  of  the  delivery  tube  is  above  the  solution 
of  potassium  iodide.  Immerse  the  closed  end  of  the  receiver  in 
cold  water  and  shake  up  the  liquid  from  time  to  time  to  hasten 
the  absorption  of  iodine  vapors.  Empty  the  receiver,  introduce 
a  fresh  portion  of  iodide  solution,  connect  up  the  apparatus, 
and  heat  again  in  order  to  ascertain  whether  all  of  the  chlorine 
was  distilled  over  in  the  first  operation.  Determine  the  iodine 
with  the  standard  solution  of  thiosulphate. 

The  following  equations  represent  the  reactions  which  are 
involved  in  the  determination: 

K2Cr207  +  14  HC1  =  2  KC1  +  2  CrCl3  +  7  H2O  +  3  Cl, 
3  CL  +  6  KI  =  6  KC1  +  3  I9. 


Any  other  oxygen   compound  which  liberates  from  hydro- 
chloric acid  a  definite  quantity  of  chlorine  can  be  determined  in 


SILVER  AND  THE  HALOGENS  229 

the  same  manner  provided  it  is  free  from  substances  which,  like 
iron  in  ferrous  salts,  absorb  chlorine  or  convert  it  into  chlorides. 

IV.    IODOMETRIC  DETERMINATION  OF  ARSENIOUS  ACID 

Weigh  from  0.1  to  0.2  gram  of  pure  resublimed  arsenious 
oxide,  and  from  0.2  to  0.4  gram  of  pure  potassium  bichromate, 
into  the  chlorine  evolution  flask.  Add  concentrated  hydro- 
chloric acid  and  a  piece  of  magnesite.  Put  the  same  quantity 
of  potassium  iodide  solution  into  the  receiver  as  before.  Con- 
nect the  two  parts  of  the  apparatus,  and  allow  the  mixture  to 
stand  for  an  hour  or  more  before  heating  and  then  proceed  as 
directed  under  III. 

The  difference  between  the  quantity  of  iodine  found  and  the 
quantity  which  the  known  weight  of  bichromate  would  have 
yielded  in  the  absence  of  any  reducing  substance  is  due  to  the  fol- 
lowing reaction,  by  which  the  arsenious  oxide  has  been  converted 
into  arsenic  acid  at  the  expense  of  a  part  of  the  chlorine : 

As2O3  +  2  H2O  +  2  C12  =  As2O5  +  4  HC1. 

There  is  therefore  a  deficit  of  four  atoms,  or  503.56  parts,  of 
iodine  for  each  molecule,  or  196.54  parts,  of  the  arsenious  oxide. 

EXERCISE  XVIII 

DETERMINATION  OF  HYPOCHLOROUS  ACID 
I.    BY  WAGNER'S  METHOD 

Weigh  about  5  grams  of  bleaching  powder  —  taking  care  to 
have  the  weighing  glass  tightly  closed  while  in  the  balance 
case  —  into  a  porcelain  mortar  and  grind  the  material  fine  with 
a  little  water.  Add  more  water  and  transfer  the  liquid  through 
a  funnel  to  a  half-liter  measuring  flask.  Grind  what  remains 
in  the  mortar  again  with  water,  etc.,  until  all  the  material  has 
been  brought  into  the  flask.  Fill  to  the  mark  with  water. 
Shake  up  the  contents  of  the  flask,  and,  without  giving  the 


230  QUANTITATIVE  EXERCISES 

suspended  matter  time  to  subside,  measure  10  cc.  of  the  liquid 
into  a  beaker  with  a  pipette.  Add  about  100  cc.  of  water  and 
6  cc.  of  the  usual  solution  of  potassium  iodide.  Acidify  with 
dilute  hydrochloric  acid,  and  determine  the  iodine  with  the 
standard  solution  of  sodium  thiosulphate. 

The  iodine  liberated  is  equivalent  to  the  active  chlorine  in 
the  bleaching  powder,  i.e.  the  chlorine  which  would  be  liberated 
if  the  material  were  treated  with  any  acid : 

CaCl20  +  2  HI  =  CaCl2  +  H2O  + 12, 
CaCl2O  +  2  HC1  =  CaCl2  +  H2O  +  C12,  or 
CaCl20  +  H2S04  -  CaS04  +  H2O  +  C12. 

There  are  two  ways  of  stating  the  quantity  of  active  chlorine 
in  bleaching  powder.  One  gives  the  volume  of  the  chlorine, 
under  standard  conditions  of  temperature  and  pressure,  which 
a  unit  weight  of  the  powder  will  yield,  and  the  other  the  per- 
centage of  the  same  by  weight. 

A  specimen  which  yields  100  cc.  of  chlorine  per  gram,  or  100 
liters  per  kilogram,  of  the  material  would  be  said  to  have 
100  chlorometric  degrees,  or  100  degrees  Gay-Lussac.  A  cubic 
centimeter  -of  chlorine  weighs  3.16742  milligrams  at  latitude 
45°  and  sea  level.  A  100-degree  bleaching  powder  must  there- 
fore contain  31.67  per  cent  of  active  chlorine.  The  results  of 
the  determination  should  be  stated  in  both  ways. 

II.    BY  PENOT'S  METHOD 

There  are  required  for  this  experiment : 

1.  Starch-iodide  papers  for  the  detection  of  hypochlorous 
acid.  Grind  together  in  a  porcelain  mortar  1.5  grams  of  starch 
and  125  cc.  of  water.  Boil  the  mixture  for  a  few  minutes. 
Add  0.5  gram  of  potassium  iodide  and  an  equal  weight  of  crys- 
tallized sodium  carbonate.  Dilute  with  water  to  250  cc.  Satu- 
rate strips  of  white  filter  paper  with  the  solution  and  dry  them. 
Keep  the  papers  in  a  closed  vessel. 


SILVER  AND  THE  HALOGENS  231 

2.  A  standard  solution  of  sodium  arsenite.     To  prepare  this, 
dissolve  2.211  grams  of  pure  arsenious  oxide  in  300  to  350  cc. 
of  hot  water  to  which  6.5  grams  of  pure  crystallized  sodium  car- 
bonate have  been  added.     Dilute  the  solution,  when  cold,  to 
one  half-liter.     Each  cubic  centimeter  of  the  solution  is  equiva- 
lent to  one  cubic  centimeter  of  chlorine  gas  when  measured 
under  standard  conditions  of  temperature  and  pressure,  or  to 
3.16742  milligrams  of  the  same. 

3.  A  solution  of  the  bleaching  powder  which  is  to  be  tested. 
This  is  prepared  in  the  same  manner  as  that  which  was  used 
with  Wagner's  method.     It  is  convenient,  however,  to  employ 
a  solution  one  cubic  centimeter  of  which  contains  0.010  gram 
of  the  original  material.    In  all  cases  the  solution  should  be 
well  shaken  just  before  measuring  out  the  portion  which  is  to 
be  tested.    „ 

Measure  into  a  beaker  the  quantity  of  the  solution  which 
contains  0.5  gram  of  bleaching  powder.  Titrate  into  it,  slowly 
and  with  constant  stirring,  the  standard  solution  of  sodium 
arsenite  until  a. drop  of  the  liquid  taken  out  with  a  glass  rod 
and  applied  to  the  starch-iodide  paper  no  longer  gives  a  blue 
color.  If  exactly  one  half-gram  of  the  material  was  employed 
for  the  determination,  the  number  of  cubic  centimeters  of  the 
sodium  arsenite  used,  multiplied  by  2,  is  equal  to  the  number 
of  the  degrees  Gay-Lussac  of  the  sample ;  and  the  number  of 
cubic  centimeters  used,  multiplied  by  3.16742,  gives  the  number 
of  milligrams  of  active  chlorine  found. 

Another  method  of  determining  the  active  chlorine  in  bleach- 
ing powder,  which  is  rapid  and  fairly  accurate,  is  that  proposed 
by  Lunge.  It  depends  upon  the  fact  that  when  a  hypochlorite 
and  hydrogen  peroxide  are  brought  together,  a  volume  of  oxy- 
gen is  evolved  which  is  exactly  equal  to  that  of  the  available 
chlorine  in  the  hypochlorite : 

CaCl20  +  H202  =  CaCl2  +  O2, 
CaCl20  +  H2S04  =  CaS04  +  H2O  +  C12. 


232  QUANTITATIVE  EXERCISES 

The  measurement  of  the  liberated  oxygen  is  therefore  equiva- 
lent to  a  measurement  of  the  chlorine  which  would  be  evolved 
if  the  material  were  treated  with  an  acid  as  represented  in  the 
second  equation. 

The  determination  is  made  by  placing  a  known  weight  of  the 
bleaching  powder  —  prepared  with  water,  as  previously  directed 
—  in  a  small  flask  and  then  lowering  into  the  flask  a  flat- 
bottomed  tube  containing  an  excess  of  hydrogen  peroxide. 
The  flask  is  attached  to  a  Hempel  burette,  or  to  some  other 
suitable  gas-measuring  tube,  which  is  partly  filled  with  water. 
The  height  of  the  water  in  the  burette  is  read,  the  tube  con- 
taining the  peroxide  overturned,  and  the  increase  in  the  volume 
of  the  gas  determined. 

It  will  be  seen  that  by  reversing  the  process  —  i.e.  by  adding 
an  excess  of  hypochlorite  to  a  known  volume  of  hydrogen  per- 
oxide—  the  strength  of  any  solution  of  the  peroxide  may  be 
determined  by  means  of  the  same  reaction.  As  a  matter  of  fact, 
the  method  is  rather  more  satisfactory  for  the  determination 
of  hydrogen  peroxide  than  for  the  determination  of  a  hypochlo- 
rite, because  the  evolution  of  oxygen  in  the  latter  case  does 
not  wholly  cease  with  the  destruction  of  the  hypochlorite.  The 
continued  liberation  of  oxygen  is  due  to  a  so-called  catalytic 
decomposition  of  the  excess  of  peroxide.  It  is,  however,  so 
slow  that  fair  determinations  of  the  hypochlorite  may  be  made 
in  spite  of  it. 

EXERCISE  XIX 

DETERMINATION   OF   HALOGENS  IN  ORGANIC   COMPOUNDS 
I.    BY  THE  LIME  METHOD 

In  a  narrow  combustion  tube,  about  400  mm.  in  length  and 
closed  at  one  end,  place  a  layer  60  mm.  in  length  of  pulverized 
lime  which  has  been  prepared  from  marble  and  is  free  from  chlo- 
rine. Upon  this  place  about  0.2  gram  of  pure  chloral  hydrate. 
Add  another  layer  of  lime  of  about  the  same  length  as  the  first. 


SILVER  AND   THE   HALOGENS  233 

Mix  the  materials  with  a  wire,  and  then  fill  the  tube  nearly  full 
of  the  lime.  Hold  the  tube  in  a  horizontal  position  and  tap  it 
upon  the  table  until  there  is  formed  at  the  top  a  channel  of  suffi- 
cient size  for  the  escape  of  gases.  Place  the  tube  in  a  combustion 
furnace  and  heat  the  lime  near  the  open  end  to  redness.  Then, 
beginning  at  the  heated  end,  light  the  other  burners  under  the 
tube,  one  after  another,  and  at  considerable  intervals  of  time. 
Having  kept  the  whole  tube  at  a  red  heat  for  twenty  minutes  or 
half  an  hour,  cool  it  slowly  and  uniformly.  Wipe  the  outside 
and  transfer  the  contents  to  a  beaker  containing  considerable 
water.  Rinse  the  tube  with  water  to  which  a  little  nitric  acid 
has  been  added.  Treat  the  contents  of  the  beaker  with  dilute 
nitric  acid  (free  from  chlorine)  until  the  liquid  becomes  perma- 
nently acid.  It  is  not  necessary  to  dissolve  the  whole  of  the 
solid  material.  The  lime  usually  contains  some  silicate  which  is 
not  readily  decomposed  by  the  acid.  Filter  through  a  small 
paper,  and  determine  the  chlorine  in  the  filtrate,  either  gravi- 
metrically  or  by  the  method  of  Volhard. 

Instead  of  emptying  and  cleansing  the  tube  in  the  manner 
prescribed,  it  may  be  closed  with  a  stopper  and  placed,  still  hot, 
in  a  high  beaker  which  is  two-thirds  full  of  cold  water.  In  this 
way,  it  will  be  broken  into  small  pieces,  which,  together  with 
the  carbon  and  other  insoluble  material,  are  filtered  out  after  the 
treatment  with  dilute  nitric  acid. 

If,  as  often  happens,  the  lime,  notwithstanding  its  preparation 
from  marble,  contains  chlorine,  the  percentage  of  the  latter  must 
be  determined  and  the  lime  employed  in  the  experiment  must 
be  weighed.  The  chlorine  belonging  to  the  lime  may  then  be 
deducted  from  the  total  found. 

If  compounds  containing  considerable  nitrogen  are  decomposed 
by  lime,  there  is  a  formation  of  cyanogen,  and  the  precipitate 
obtained  with  silver  nitrate  is  then  a  mixture  of  chloride  and 
cyanide.  The  formation  of  cyanogen  may  be  avoided  by  decom- 
posing nitrogenous  halogen  compounds  with  soda-lime  instead 
of  lime. 


234  QUANTITATIVE  EXERCISES 

Bromine  and  iodine  are  determined  by  the  lime  method  in  the 
same  manner  as  chlorine.  It  is  necessary,  however,  especially 
in  the  determination  of  iodine,  to  add  a  little  sulphurous  acid  to 
the  solution  before  precipitating  with  silver  nitrate. 

Liquid  substances  are  weighed  and  introduced  into  the  com- 
bustion tube  in  small  bulbs  blown  on  the  ends  of  capillary  tubes. 
The  bulbs  should  be  capable  of  holding  about  twice  the  quantity 
of  liquid  which  is  required  for  the  analysis,  and  the  tubes  to 
which  they  are  attached  should  be  from  75  to  100  mm.  in 
length.  After  weighing  the  bulb,  the  outlet  is  immersed  in  the 
liquid,  which  may  then  be  made  to  rise  into  the  bulb  by  alter- 
nately heating  and  cooling  the  bulb  or  by  placing  the  vessel  con- 
taining the  liquid  under  the  bell  of  an  air  pump  and  exhausting. 
The  bulb  is  wiped  dry,  closed  by  holding  the  open  end  in  the 
flame  of  a  Bunsen  burner,  and  weighed.  Before  introducing  it 
into  the  combustion  tube,  the  capillary  tube  is  scratched  with 
a  file  at  a  little  distance  from  the  base,  and  broken  off.  Both 
parts  are  then  dropped  into  the  lime  or  soda-lime  in  the  bottom 
of  the  combustion  tube. 


II.   BY  CARIUS'  METHOD 

In  a  thick  combustion  tube  —  having  a  length  of  400  or 
500  mm.  and  an  internal  diameter  of  13  to  15  mm.,  and  closed 
at  one  end  —  place  from  2.5  to  3  cc.  of  nitric  acid  of  1.5  specific 
gravity.  In  a  narrow  tube,  75  or  100  mm.  long,  weigh  about 
0.2  gram  of  chloral  hydrate,  and  add  to  it  about  four  times  its 
weight  of  crystallized  silver  nitrate.  Place  the  smaller  tube  in 
the  larger  one  and  draw  out  the  open  end  of  the  latter  to  a  capil- 
lary with  thick  walls,  taking  care  not  to  let  the  nitric  acid  and 
the  substance  come  together  until  the  tube  is  sealed.  The  explo- 
sions which  so  often  occur  when  substances  are  heated  in  closed 
tubes  are  usually  due  to  a  faulty  sealing  of  the  tubes.  It  is 
well,  therefore,  for  one  not  experienced  in  this  kind  of  manipu- 
lation to  practice  for  a  time  —  under  supervision  if  possible  — 


SILVER  AND  THE  HALOGENS  235 

with  odd  pieces  of  glass  tubing  before  attempting  to  seal  the 
tube  which  is  to  be  used  in  this  exercise.  It  is  important  that 
the  wall  of  the  drawn-out  portion  of  the  tube  should  be  made 
quite  thick  at  every  point,  and  the  glass  should  be  well  annealed 
by  turning  it  in  the  smoky  flame  until  it  is  densely  and  uniformly 
covered  with  carbon. 

Put  the  closed  tube  in  an  iron  one,  letting  the  capillary  end 
project  a  little  beyond  the  mouth  of  the  latter.  Place  the  two 
tubes  in  the  ubomb"  furnace  and  put  an  empty  wooden  box 
before  it  to  receive  the  material  which  will  be  ejected  if  an 
explosion  occurs.  Gradually  raise  the  temperature  of  the  fur- 
nace to  250°,  and  keep  it  there  for  two  hours.  When  the  glass 
tube  has  cooled  to  the  temperature  of  the  room,  take  the  capil- 
lary end  between  the  thumb  and  forefinger  and  draw  the  tube 
forward  under  a  folded  towel ;  and  as  soon  as  it  is  out  of  the 
iron  tube  roll  it  up  in  the  towel.  The  opening  of  the  tube, 
which  requires  great  caution,  is  best  accomplished  in  the  follow- 
ing manner:  A  300-  or  400-cc.  beaker,  partly  rilled  with  water, 
is  tilted  forward  and  secured  in  this  position  by  a  block ;  a  burner 
is  placed  directly  in  front  of  the  beaker;  finally,  the  capillary 
end  of  the  tube  is  uncovered  and  cautiously  heated  until  the 
glass  softens,  swells  out,  and  opens  for  the  escape  of  the  com- 
pressed gases  within.  The  issuing  material  should,  of  course, 
be  directed  into  the  beaker.  Cut  the  tube  near  the  middle,  — 
taking  care  that  no  fragments  of  glass  fall  into  either  part,  — 
wash  the  contents  into  a  beaker,  nearly  neutralize  the  excess  of 
nitric  acid  with  sodium  carbonate  which  is  free  from  chlorine, 
and  determine  the  silver  chloride  in  the  usual  manner. 


Bromine  and  iodine  in  organic  compounds  may  also  be  deter- 
mined by  the  method  of  Carius.  The  author  of  the  method 
recommends  in  the  case  of  iodine  compounds  that  the  mixture 
of  silver  iodide  and  nitrate  be  heated  until  it  fuses  to  a  yellow 
liquid  which  solidifies  on  cooling  to  an,  opaque  yellow  mass. 


236  'QUANTITATIVE  EXERCISES 

This  must  afterwards  be  heated  in  the  diluted  liquid  from  one 
to  two  hours  in  order  to  effect  a  separation  of  the  iodide  and 
nitrate. 

The  halogens  may  be  removed  from  many  organic  compounds 
by  treating  their  aqueous  or  alcoholic  solutions  with  sodium 
amalgam. 

The  Separation  of  the  Halogens 

Any  two  of  the  halogens,  when  the  third  is  absent,  may  be 
determined  by  an  indirect  method.  Suppose,  for  example,  that 
chlorine  and  bromine,  in  the  form  of  a  chloride  and  a  bromide,  are 
in  solution  and  are  to  be  determined  indirectly.  The  best  pro- 
cedure is  as  follows :  The  amount  of  silver  required  for  the 
precipitation  of  both  is  ascertained  by  the  volumetric  method 
of  Mohr;  the  precipitate  of  AgCl  and  AgBr  is  then  collected, 
washed,  dried,  and  weighed.  If,  for  any  reason,  the  method  of 
Volhard  is  to  be  preferred  to  that  of  Mohr,  the  solution  is 
diluted  to  a  known  volume  and  divided,  one  measured  portion 
being  used  for  the  volumetric,  and  the  remainder  for  the  gravi- 
metric, determination. 

The  calculation  is  as  follows : 

Atomic  and  Molecular  Weights 

Ag  =  1 07.11,  AgCl  =  142.29, 

Cl  =  35.18,  AgBr  =  186.46. 

Br  =  79.35, 

Let  x  =  weight  of  chlorine, 
y  =       "       "  bromine, 

a  =       "       "  silver  required  to  precipitate  Cl  and  Br, 
b=       «       "  AgCl  +  AgBr. 
Then  x  +  y  =  b  —  a,  and 

r *-*•    4-04462,+ 2.34984,  =  i; 

x  =  1.38651  a  -  0.79646  b,  and 
y  =  1.79646  b  -2.38651  a. 


SILVER  AND  THE   HALOGENS  237 

Several  methods  have  been  proposed  for  the  separation  of 
bromine  from  chlorine,  all  of  which  depend  upon  the  fact  that 
certain  oxidizing  agents  under  certain  conditions  are  capable 
of  liberating  bromine  from  bromides  but  not  chlorine  from 
chlorides.  The  bromine,  when  liberated,  is  expelled  from  the 
solution,  —  with  the  aid  usually  of  a  current  of  steam  or  air, 
—  and  is  collected  either  in  a  solution  of  some  alkali  to  which 
has  been  added  a  quantity  of  hydrogen  peroxide  in  order  to 
prevent  the  formation  of  a  hypobromite,  or  in  a  solution  of 
potassium  iodide.  In  the  latter  case  the  equivalent  quantity 
of  liberated  iodine  is  determined  volumetrically  by  means  of 
sodium  thiosulphate. 

Some  of  the  oxidizing  agents  which  have  been  employed  for  the 
liberation  of  bromine  in  a  mixture  of  bromides  and  chlorides  are : 

1.  Lead  peroxide  and  acetic  acid  (Vortmann*). 

2.  Potassium   permanganate    and   acid    potassium    sulphate 
(Berglund  f). 

3.  Potassium  permanganate  and  aluminium  sulphate  (White  £). 

4.  Potassium  permanganate  and  acetic  acid  (Jannasch  and 
Aschon°§). 

5.  Potassium  bichromate  and  sulphuric  acid  (Dechan,  ||  Fried- 
heim  and  Meyer  **). 

6.  Ammonium  persulphate  (Engel  ff ). 

7.  Potassium  permanganate  and  ferrous  sulphate  (Weiss  ££). 

8.  lodic  acid  (Bugarszky  §§). 

9.  Potassium  permanganate  in  the  presence  of  copper  salts 
(Baubingy  and  Rivals  ||  ||). 

*  Zeitsch.  anal.  Chem.,  19,  352 ;  22,  565,  566 ;  24,  196;  25,  172. 

t  Ibid.,  24,  184. 

J  Chem.  News,  57,  283  ;  58,  229. 

§  See  references  cited  under  Exercise  XX. 

||  Jour.  Chem.  Soc.,  49,  682. 
**  Zeitsch.  anorg.  Chem.,  1,  407. 
ft  Zeitsch.  anal.  Chem.,  34,  616. 
U  Ibid.,  34,  615. 

§§  Zeitsch.  anorg.  Chem.,  10,  387. 
(Ill  Comptes  Rendus,  124,  859;  125,  527,  607. 


238  QUANTITATIVE  EXERCISES 

Iodine  may  be  separated  from  chlorine  and  bromine  in  a  mix- 
ture of  halogen  salts  in  various  ways : 

1.  It  may  be  liberated  by  nitrous  acid,*  which  does  not  decom- 
pose hydrochloric  or  hydrobromic  acid,  and  then  removed  from 
the  solution,  either  by  extraction  with  carbon  bisulphide,  or  by 
passing  a  current  of  steam  through  the  liquid.     If  extracted  by 
carbon  bisulphide,  it  is  best  determined  by  a  standard  solution 
of  sodium  thiosulphate.     If,  on  the  other  hand,  the  liberated 
iodine  is  removed  by  steam,  it  is  best  to  conduct  the  vapors  into 
an  alkaline  solution  of  hydrogen  peroxide,  and  to  determine  it, 
either  volumetrically  or  gravimetrically,  as  silver  iodide. 

2.  The  iodine  may  be  liberated  by  arsenic  acid  (Gooch  and 
Browningf)  and  removed  by  evaporation  and  the  resulting  equiv- 
alent quantity  of  arsenious  acid  determined  volumetrically  as 
recommended  by  the  authors  of  the  method ;  or  it  may  be  removed 
with  the  aid  of  steam,  collected  in  a  solution  of  potassium  iodide, 
and  determined  by  sodium  thiosulphate  as  recommended  by 
Friedheim  and  Meyer.  J 

3.  The  iodine  maybe  precipitated  by  thallium  nitrate  (Hiibner, 
Spezia,  and  Frerichs  §) ;  or,  if  the  quantity  of  bromine  is  small, 
by  palladium  nitrate. 

In  the  absence  of  bromine,  iodine  may  be  separated  from 
chlorine  by  precipitating  with  thallium  sulphate  in  the  presence 
of  ammonium  sulphate  and  alcohol  (Jannasch  and  Aschoff  ||). 

*  See  Fresenius,  Quant.  Analyse,  1,  659;  also  Gooch  and  others,  Zeitsch. 
anal  Chem.,  30,  58;  34,  603. 

t  Zeitsch.  anal.  Chem.,  30,  60;  Zeitsch.  anorg.  Cftem.,4,  178. 

|  Zeitsch.  anorg.  Chem.,  1,  407. 

§  Fresenius,  Quant.  Analyse,  1,  661. 

||  Zeitsch.  anorg.  Chem.,  \ ,  248, 


SILVER  AND  THE   HALOGENS 


239 


EXERCISE  XX 

THE  SEPARATION  OF  CHLORINE,  BROMINE,  AND  IODINE 
BY  THE  METHOD  OF  JANNASCH  AND  ASCHOFF  * 

The  apparatus  which  is  recommended  by  the  authors  of  the 
method  is  shown  in  Fig.  46. 
It  consists  of : 

1.  A  metallic  vessel  «,  in  which  steam  is  produced  for  the 
expulsion  of  iodine  and  bromine. 

2.  The  flask  6,  with  a  capacity  of  about  one  liter,  in  which 
the  iodine  and  bromine  are  successively  liberated  —  the  former 
by  nitrous  acid  and 

the  latter  by  potas- 
sium permanganate 
and  acetic  acid. 

3.  The   thin  flask 
c,  with  a  capacity  of 
about  500  cc.,  which 
contains  a  solution  of 
sodium  hydroxide 

and  hydrogen  peroxide,  and  is  cooled  by  running  water. 

4.  The  Peligot  tube  d,  which  also  contains  a  small  quantity 
of  the  alkaline  solution  of  hydrogen  peroxide. 

5.  The  Erlenmeyer  flask  e,  containing  a  dilute  solution  of 
caustic  soda  to  which  a  little  ammonia  has  been  added. 

The  required  reagents  are : 

1.  A  10  per  cent  solution  of  sodium  nitrite. 

2.  A  5  per  cent  solution  of  sodium  hydroxide. 

3.  A  3  per  cent  solution  of  hydrogen  peroxide. 

4.  A  30  per  cent  solution  of  pure  acetic  acid. 

5.  A  nearly  saturated  solution  of  potassium  permanganate 
(approximately  5  per  cent). 


FIG.  46 


*  Zeitsch.  anorg.  Chem.,  1,  144;  5,  8. 


240  QUANTITATIVE  EXERCISES 

All  of  the  reagents  must  be  free  from  halogens,  and  the  acetic 
acid  must  be  free  from  substances  with  which  the  halogens  form 
substitution  products. 

Weigh  out  about  one  quarter  gram  each  of  pure  chloride, 
bromide,  and  iodide  of  potassium.  Introduce  the  salts  into  the 
flask  6,  dissolve  in  water,  and  dilute  to  about  750  cc. 

Measure  100  cc.  of  a  mixture  of  equal  volumes  of  the  sodium 
hydroxide  and  hydrogen  peroxide  solutions  into  the  flask  c?,  and 
a  few  cubic  centimeters  of  the  same  mixture  into  the  Peligot 
tube  d.  In  the  Erlenmeyer  flask  e  place  a  small  quantity  of 
the  caustic  soda  solution  and  add  to  it  a  little  ammonia. 

Having  brought  the  water  in  a  to  the  boiling  point,  add  to 
the  solution  of  the  salts  first  5  cc.  of  dilute  sulphuric  acid,  and 
then  10  cc.  of  the  solution  of  sodium  nitrite.  Close  the  flask 
quickly  and  conduct  steam  through  it  until  all  of  the  iodine  has 
been  removed,  heating  the  contents  of  the  flask  in  the  mean- 
time with  a  small  flame. 

Disconnect  the  absorption  apparatus  while  the  steam  is  still 
passing  through,  and  transfer  the  contents  of  c  and  d  quantita- 
tively to  a  porcelain  dish,  keeping  the  liquid  in  e  separate. 

Make  the  solution  of  chloride  and  bromide  in  b  distinctly  alka- 
line with  caustic  soda  and  concentrate  it  in  a  porcelain  dish  to  a 
volume  of  50  cc.  Return  the  solution  to  b  and  allow  it  to  cool. 
Arrange  and  refill  the  absorption  apparatus  as  for  the  collection 
of  iodine,  and  introduce  into  b  60  cc.  of  the  30  per  cent  acetic 
acid  and  the  volume  of  the  permanganate  solution  which  con- 
tains 1  gram  of  the  salt.  Heat  the  flask  with  a  small  flame  and 
pass  steam  through  it  for  not  less  than  an  hour  after  the  con- 
tents have  reached  the  boiling  point. 

Determination  of  the  Iodine 

Heat  the  liquid  derived  from  c  and  d — which  probably  con- 
tains all  of  the  iodine  —  for  a  long  time  in  a  covered  porcelain 


SILVER  AND  THE   HALOGENS  241 

dish,  add  50  cc.  of  the  hydrogen  peroxide  solution  to  complete 
the  oxidation  of  any  sodium  nitrite  which  may  be  present. 
Afterwards  add  the  contents  of  the  Erlenmeyer  flask  e,  and 
concentrate  to  a  suitable  volume.  To  the  alkaline  solution  add 
silver  nitrate,  little  by  little,  and  stir  until  the  brown  oxide  of 
silver  which  is  first  formed  is  no  longer  converted  into  the 
yellow  iodide.  Acidify  with  nitric  acid  and  heat  for  some  hours 
upon  the  water  bath,  or  until  the  precipitate  has  properly  sub- 
sided. Filter  hot,  wash  with  boiling  water,  etc. 

Determination  of  the  Bromine 

Heat  the  contents  of  c  and  d  in  a  covered  porcelain  dish  with 
an  additional  quantity  of  hydrogen  peroxide  in  order  to  destroy 
any  sodium  hypobromite  which  may  be  present.  Add  the  liquid 
in  the  Erlenmeyer  flask  and  concentrate.  Precipitate  the  bro- 
mine with  a  mixture  of  equal  parts  of  10  per  cent  silver  nitrate 
solution  and  concentrated  nitric  acid.  Heat  from  one  to  two 
hours  on  the  water  bath,  filter  hot,  wash  with  boiling  water,  etc. 

Determination  of  the  Chlorine 

Reduce  the  excess  of  permanganate  with  an  ammoniacal  solu- 
tion of  hydrogen  peroxide.  Collect  the  oxide  of  manganese 
upon  a  roomy  filter  and  wash  with  a  one  per  cent  solution  of 
sodium  nitrate.  Determine  the  chlorine  in  the  filtrate  as  silver 
chloride. 


If  the  liquid  in  which  the  three  halogens  are  to  be  determined 
contains  calcium,  the  use  of  sulphuric  acid  to  decompose  the 
nitrite  for  the  purpose  of  liberating  iodine  is  inadmissible.  In 
such  a  case  the  authors  of  the  method  acidify  the  solution  with 
a  few  drops  of  acetic  acid  and  then  conduct  into  the  flask  from 
the  outside  the  vapors  of  nitrous  acid  until  the  space  above  the 
liquid  begins  to  fill  with  the  yellowish-red  gas. 


242  QUANTITATIVE  EXERCISES 

Jannasch  and  Koelitz*  have  found  that  when  the  halogens 
are  to  be  determined  in  an  alkaline  liquid,  either  sulphuric  or 
nitric  acid  —  and  not  acetic  acid  —  should  be  employed  to  neu- 
tralize the  solution,  since  the  presence  of  acetates  —  but  not 
of  sulphates  or  nitrates  —  retards  the  quantitative  liberation  of 
bromine. 

To  determine  the  halogens  in  organic  compounds  the  authors  f 
proceed  as  follows : 

If  the  substance  is  to  be  decomposed  by  the  "  lime  method," 
a  hard  glass  tube  —  about  50  centimeters  long  and  having  an 
internal  diameter  of  4  mm.  —  is  filled  to  a  depth  of  3  or  4 
centimeters  with  quicklime.  The  material  —  previously  mixed 
with  lime  in  a  deep  porcelain  mortar  —  is  then  introduced. 
Afterwards  the  mortar,  funnel,  etc.,  are  rinsed  a  number  of 
times  with  small  portions  of  finely  ground  lime,  which  are  like- 
wise filled  into  the  tube  until  there  remains  room  only  for  a 
plug  of  asbestus.  After  heating  in  the  usual  manner,  the  cooled 
contents  of  the  tube  are  transferred  to  a  glass-stoppered  liter 
flask  containing  about  300  cc.  of  water.  The  tube  is  rinsed  first 
with  water  and  then  with  dilute  nitric  acid.  Strong  nitric  acid, 
in  small  quantities,  is  then  added  until  only  a  small  portion  of 
the  lime  remains  undissolved,  care  being  taken  constantly  to 
agitate  the  contents  of  the  flask  and  to  keep  them  cool.  If, 
accidentally,  the  whole  of  the  lime  is  dissolved,  more  must  be 
added  in  order  to  produce  an  alkaline  condition.  After  filtering 
and  washing  the  solid  residue  with  hot  water,  the  halogens  are 
precipitated  with  a  mixture  consisting  of  equal  parts  of  concen- 
trated nitric  acid  and  a  10  per  cent  silver  nitrate  solution.  The 
silver  salts-  are  heated  upon  a  water  bath,  with  frequent  agita- 
tion, for  an  hour  or  more,  or  until  the  precipitate  has  fully  sub- 
sided, and  then  collected  upon  a  small  paper  filter.  The  washed 
contents  of  the  filter  —  together  with  the  still  moist  paper  —  are 
transferred  to  a  silver  crucible  and  fused  with  5  or  6  grams  of 
pure  sodium  hydroxide  until  the  paper  is  fully  burned  and  a 
*  Zeitsch,  anorg.  Chem.,  15,  66.  t  Ibid.,  15,  68. 


SILVER  AND  THE  HALOGENS  243 

tranquil  liquid  is  obtained.  Finally,  the  residue  is  treated  with 
water,  warmed  upon  a  water  bath,  filtered,  the  excess  of  alkali 
neutralized  with  sulphuric  or  nitric  acid,  and  the  halogens  sepa- 
rated in  the  manner  prescribed  in  Exercise  XX. 

The  procedure  with  the  silver  salts  is  identical  with  that 
described  above  when  the  organic  compound  is  decomposed  by 
the  method  of  Carius.  They  are  collected  upon  a  paper,  washed, 
transferred  —  still  wet  —  to  a  silver  crucible,  fused  with  sodium 
hydroxide,  etc. 


CHAPTER  X 
SULPHUR 


EXERCISE  XXI 

THE  DETERMINATION  OF  SULPHURIC  ACID  IN  BARIUM 

SULPHATE 

Weigh  in  a  platinum  crucible  from  0.3  to  0.4  gram  of  pure, 
finely  pulverized  and  dried  heavy  spar.  Add  about  five  times  as 
much  sodium-potassium  carbonate  which  is  free  from  sulphates, 
and  fuse  over  the  blast  lamp  until  the  decomposition  is  believed 
to  be  complete.  Place  the  crucible  on  its  side  in  a  porcelain 
dish,  cover  it  with  distilled  water,  and  heat  until  the  excess  of 
the  carbonate  and  the  alkali  sulphates  are  dissolved.  Filter,  and 
wash  the  barium  carbonate  with  water  to  which  a  little  ammonia 
and  ammonium  carbonate  have  been  added.  Slightly  acidify  the 
filtrate  with  hydrochloric  acid.  Dilute,  if  necessary,  to  a  volume 
of  100  or  150  cc. ;  heat  nearly  to  the  boiling  point,  and  precipi- 
tate the  sulphuric  acid  (slowly  and  with  constant  stirring)  with 
a  dilute  hot  solution  of  barium  chloride,  taking  care  to  add  a 
slight  excess  of  the  latter.  Continue  to  heat  the  liquid  for  an 
hour,  ^at  least,  and  then  put  aside  to  settle. 

When  the  liquid  above  the  precipitate  has  become  perfectly 
clear,  pour  it  through  a  small  filter.  Treat  the  precipitate 
remaining  in  the  beaker  with  boiling  water  containing  a  little 
hydrochloric  acid,  and  pour  this  also  through  the  paper  when 
the  barium  sulphate  has  subsided.  Wash  the  precipitate  several 
times  in  this  manner,  and  then  bring  it  upon  the  filter.  Wash 
again  with  hot  water  until  the  filtrate  gives  no  reaction  for 
chlorine. 

244 


SULPHUR  245 

Detach  the  edge  of  the  filter  from  the  glass  with  the  aid  of  a 
stout  platinum  wire  flattened  at  one  end,  or  a  thin  spatula,  and 
lift  it  out  of  the  funnel.  Examine  the  glass  carefully  for  pre- 
cipitate which  may  have  crept  over  the  edge  of  the  paper,  and, 
if  any  is  found,  wipe  it  up  with  a  fragment  of  ashless  paper 
which  is  afterwards  to  be  placed  in  the  filter.  Fold  the  filter, 
without  drying,  into  a  compact  wad,  and  place  it  in  a  weighed 
platinum  crucible.  The  subsequent  incineration  requires  some 
caution.  The  crucible,  tilted  to  one  side,  is  placed  in  a  triangle 
which  rests  upon  the  ring  of  an  iron  stand.  At  first  the  ring 
is  raised  considerably  above  the  flame,  where  it  is  kept  until  the 
paper  is  dry.  Afterwards  it  is  lowered  from  time  to  time  until 
the  charring  process  is  finished,  but  never  so  far  that  the  volatile 
products  burst  into  flames.  Finally,  when  nothing  remains  of 
the  paper  except  carbon,  the  crucible  is  lowered  into  the  flame 
and  the  incineration  completed.  The  residue  of  carbon  should 
burn  quite  readily,  leaving  a  perfectly  white  ash.  If  it  does  not, 
measures  should  be  taken  to  promote  the  circulation  of  air  over 
the  heated  material.  To  this  end  the  lamp  is  so  placed  that 
only  the  bottom  of  the  tilted  crucible  is  in  contact  with  the 
flame ;  and  the  lid,  with  its  edge  resting  on  the  triangle,  is 
allowed  to  lean  against  the  mouth  of  the  crucible.  It  is  quite 
practicable,  of  course,  in  cases  of  difficult  incineration  to  intro- 
duce into  the  crucible  a  current  of  air  or  of  oxygen  through  a 
small  glass  tube. 

It  is  recommended  by  Richards,  in  precipitating  sulphuric 
acid  by  barium  chloride,  to  allow  the  solution  of  the  latter  to 
flow  in  a  thin  stream  down  the  side  of  the  beaker  containing 
the  former.  The  liquid  spreads  out  in  its  passage  over  the 
glass  wall,  thus  giving  a  great  surface  of  first  contact  between 
the  two  solutions.  The  following  manipulation  of  this  method 
of  precipitation  has  been  found  very  satisfactory :  By  rotating 
the  beaker  in  the  hand,  the  hot  solution  of  sulphate  is  given  a 
somewhat  rapid  circular  motion.  The  beaker  is  then  placed  on 
the  sand  bath  or  wire  gauze,  and  the  hot  solution  of  chloride  is 


246  QUANTITATIVE  EXERCISES 

slowly  poured  down  the  side.  When  the  movement  of  the 
liquid  becomes  too  sluggish,  the  beaker  is  taken  from  the  bath 
and  again  rotated.  With  proper  management  a  precipitate  is 
obtained  which  subsides  completely  within  a  few  minutes. 


When  sulphuric  acid  is  precipitated  by  barium  chloride,  some 
of  the  latter  is  always  occluded  by  the  barium  sulphate,  and 
no  amount  of  washing  suffices  for  its  complete  removal.  The 
sulphate,  however,  is  somewhat  soluble,  and  under  favorable 
conditions  of  precipitation  the  two  sources  of  error  nearly  com- 
pensate each  other.  The  occlusion  is  greater  in  concentrated 
than  in  dilute  solutions.  It  is  greater  also  in  cold  than  in  hot 
solutions.  According  to  Richards  the  occlusion  is  increased 
by  the  presence  of  hydrochloric  acid,  but  not  by  the  addi- 
tion of  barium  chloride  after  the  precipitation  of  the  sulphuric 
acid  has  been  completed.  The  quantity  of  chloride  which  is 
carried  down  by  the  sulphate  is  greater  when  the  latter  is 
formed  in  the  presence  of  an  excess  of  the  former,  i.e.  when  the 
sulphuric  acid  or  soluble  sulphate  is  added  to  the  solution  of 
the  barium  salt ;  hence  in  precipitating,  whether  for  the  deter- 
mination of  sulphuric  acid  or  of  barium,  the  solution  of  the 
barium  salt  is  poured  into  that  of  the  sulphate.  This  order 
should  never  be  reversed.  The  amount  of  the  occluded  chloride 
can  be  ascertained  by  fusing  the  precipitated  sulphate  with 
an  alkaline  carbonate  and  determining  the  chlorine  as  silver 
chloride. 

The  solubility  of  barium  sulphate  varies  somewhat  with  the 
circumstances  under  which  it  is  precipitated ;  it  is  given  as  one 
part  in  400,000  parts  of  water  under  the  usual  conditions  of 
precipitation.  The  precipitation  is  retarded  even  by  very  dilute 
hydrochloric  acid,  while  in  the  presence  of  the  moderately  con- 
centrated acid  it  is  quite  incomplete.  The  presence  either  of  a 
soluble  barium  salt  or  of  a  sulphate  (anything  furnishing  an  ion 
in  common  with  it)  diminishes  its  solubility;  hence  the  practice 


SULPHUR  247 

in  this  case,  as  in  quantitative  work  in  general,  of  adding  an 
excess  of  the  precipitating  agent.  It  is  dissolved  by  concen- 
trated sulphuric  acid  and  reprecipitated  on  dilution  with  water. 
For  further  information  regarding  the  solubility  of  barium 
sulphate  see  Fresenius  and  Hintz,  Zeitschrift  fur  analytische 
Chemie,  35,  170. 

There  is  some  danger  when  the  filter  is  burned  too  rapidly, 
i.e.  at  too  high  a  temperature,  that  a  little  of  the  sulphate  will 
be  reduced  to  sulphide,  which  may  be  converted  by  the  water 
vapor  and  carbon  dioxide  surrounding  it  first  into  hydroxide 
and  afterwards  into  carbonate.  It  is  therefore  well  to  moisten 
the  material,  after  the  first  ignition  and  weighing,  with  a  drop  of 
sulphuric  acid,  and  then  to  evaporate,  ignite,  and  weigh  again. 


EXERCISE  XXII 
DETERMINATION  OF  SULPHUR  IN  SULPHIDES 

The  nitric  acid  which  is  to  be  used  in  this  and  in  a  subsequent 
experiment  is  to  be  prepared  in  the  following  manner:  Pour 
into  a  tubulated  liter-and-a-half  retort,  first  200  cc.  of  concen- 
trated nitric  acid,  and  then  600  cc.  of  concentrated  sulphuric  acid. 
Fasten  the  retort  in  a  clamp,  and  —  without  boiling  the  mixture 
—  slowly  distill  about  100  cc.  of  the  nitric  acid  into  a  receiver 
which  fits  the  retort  closely.  The  distillate  will  probably  con- 
tain sulphuric  acid,  which  must  be  removed.  Place  one  or  two 
grains  of  pulverized  and  thoroughly  dried  barium  nitrate  in  a 
glass-stoppered  bottle  and  add  the  nitric  acid  to  it.  Shake  the 
bottle  from  time  to  time  for  several  hours.  Pour  the  acid  into 
a  small  retort  and  distill  it  slowly  into  a  receiver.  The  distil- 
late must  now  be  tested  for  sulphuric  acid.  Place  about  5  cc. 
of  it  in  a  clean  dry  retort,  add  about  half  a  gram  of  pure  potas- 
sium nitrate,  and  distill  to  dryness.  Dissolve  the  residue  in  the 
retort  in  water,  add  a  drop  or  two  of  hydrochloric  acid,  and  test 
for  sulphuric  acid  with  barium  chloride. 


248  QUANTITATIVE  EXERCISES 

Solutions  which  are  to  be  tested  for  sulphuric  acid,  or  in 
which  the  presence  of  small  quantities  of  that  acid  would  be 
harmful,  must  not  be  evaporated  in  open  vessels  over  the  ordi- 
nar}^  water  bath.  Owing  to  the  presence  of  sulphur  in  the  gas, 
solutions  so  treated  almost  invariably  become  contaminated  with 
sulphuric  acid. 

Weigh  into  narrow,  flat-bottomed  tubes  two  portions  (0.2  to 
0.3  gram  each)  of  finely  ground  pyrites.  This  mineral  is  selected 
because  it  contains  iron,  which  introduces  a  certain  difficulty 
into  the  determination  of  sulphuric  acid  as  sulphate  of  barium. 
Pour  into  pressure  bottles  —  so-called  "  Lintner  bottles"  —  from 
5  to  10  cc.  of  the  nitric  acid  which  has  been  prepared  as  directed. 
Set  the  tubes  containing  the  weighed  material  in  the  bottles 
in  an  upright  position.  Fasten  the  latter  in  their  frames,  and 
then  tilt  them  so  that  the  tubes  fall  on  their  sides.  Shake  the 
bottles  gently  until  the  acid  and  the  mineral  are  well  mixed. 
When  the  reaction  ceases,  or  becomes  very  slow,  place  the 
bottles  in  a  bath  containing  cold  water.  Heat  the  water  and 
keep  it  near  the  boiling  point  for  two  hours  or  more.  Examine 
the  contents  of  the  bottles  for  undecomposed  mineral  and  unoxi- 
dized  sulphur.  The  presence  of  solid  matter,  when  sulphides 
are  treated  in  this  manner,  is  not  a  certain  evidence  that  the 
reaction  is  unfinished,  since  it  may  consist  of  sulphates  which, 
though  soluble  in  water,  are  insoluble  or  only  slightly  soluble  in 
nitric  acid.  If  the  reaction  is  incomplete,  heat  again  in  the 
water  bath  as  before.  If,  on  the  other  hand,  it  appears  to  be 
complete,  transfer  the  contents  of  the  bottles  to  porcelain  dishes. 
If  the  mineral  contained  no  insoluble  foreign  matter,  and  the 
oxidation  is  finished,  the  diluted  solutions  will  be  clear.  If 
they  are  free  from  suspended  matter,  add  to  each  about  0.3  gram 
of  pure  potassium  chloride  to  insure  the  presence  of  enough 
base  to  convert  all  of  the  sulphuric  acid  into  sulphates,  and 
evaporate  to  dryness  at  a  low  temperature  on  a  broad  sand  bath. 
Treat  the  residues  with  strong  hydrochloric  acid  and  again 
evaporate  to  dryness.  Repeat  the  addition  of  hydrochloric  acid 


SULPHUR  249 

and  the  subsequent  evaporation  until  all  the  nitric  acid  has  been 
removed.  Treat  the  residues  with  a  few  drops  of  hydrochloric 
acid  and  dissolve  them  in  water.  If  the  solutions  contain  sus- 
pended matter,  they  are  to  be  filtered. 


Some  sulphides,  when  decomposed  with  nitric  acid  in  the 
manner  just  described,  yield  a  deposit  of  free  sulphur  which  is 
only  very  slowly  converted  into  sulphuric  acid.  In  such  cases 
the  bottles  should  be  opened  when  cold  and  the  contents  treated 
with  a  little  potassium  chlorate  or  strong  hydrochloric  acid; 
after  which  they  are  to  be  closed  and  again  gently  heated.  If 
this  procedure  fails  to  complete  the  oxidation  of  the  sulphur, 
the  diluted  liquid  must  be  filtered  through  a  weighed  paper, 
and  the  filter  and  its  contents  thoroughly  washed,  dried  at  con- 
stant temperature,  and  weighed.  The  residue  on  the  paper, 
whose  weight  is  determined  in  this  manner,  will  consist  of  the 
unoxidized  sulphur  and  any  impurity  in  the  mineral  which  has 
not  been  attacked  by  the  nitric  acid.  The  paper  and  its  con- 
tents are  therefore  incinerated  and  the  ash  weighed.  The  loss 
during  incineration  is  trie  weight  of  the  unoxidized  sulphur. 

When  sulphuric  acid  is  precipitated  by  barium  chloride  in 
the  presence  of  ferric  salts,  the  barium  sulphate  carries  down 
with  it  a  quantity  of  iron,  either  as  ferric  sulphate  or  as  a 
double  sulphate  of  barium  and  iron,  and  the  iron  cannot  be 
wholly  removed  even  by  washing  the  precipitate  with  dilute 
acids ;  neither  can  the  precipitation  of  the  iron  be  prevented  by 
the  presence  of  any  quantity  of  acid  which  would  not  interfere 
with  the  completeness  of  the  precipitation  of  the  sulphuric  acid. 
\  Again,  when  barium  sulphate  which  is  contaminated  with  ferric 
sulphate  is  ignited,  the  sulphuric  acid  in  combination  with  the 
iron  is  volatilized,  and  the  barium  salt  becomes  colored  from 
pink  to  brown  by  ferric  oxide.  ;  But  the  weight  of  the  oxide  is 
not  equal  to  that  of  the  barium  sulphate  which  the  lost  acid 
should  have  formed.  It  is  therefore  found  that  the  results  are 


250  QUANTITATIVE  EXERCISES 

usually  too  low  when  sulphuric  acid  is  determined  in  the  ordi- 
nary manner  in  the  presence  of  ferric  salts.  Various  methods 
of  obviating  this  difficulty  have  been  proposed : 

(a)  Lunge  *  precipitates  the  iron  as  ferric  hydroxide  by  adding 
a  not  too  large  excess  of  ammonia  to  the  moderately  warm  solu- 
tion, filters  after  10  minutes,  washes  the  precipitate  with  boiling 
water,  and  determines  the  sulphuric  acid  in  the  filtrate  in  the 
usual  manner,  i.e.  by  precipitating  as  barium  sulphate  in  the 
presence  of  hydrochloric  acid.     If  the  ferric  hydroxide  is  sus- 
pected of  retaining  sulphuric  acid,  it  may  be  fused  with  sodium 
carbonate  in  a  platinum  crucible,  the  sodium  salts  dissolved  in 
water,  filtered,  the  filtrate  acidified  with  hydrochloric  acid,  and 
tested  with  barium  chloride. 

(b)  Fresenius  f  fuses  the  pulverized  mineral  in  which  sulphur 
is  to  be  determined  in  a  platinum  crucible  with  ten  parts  of  a  mix- 
ture consisting  of  two  parts  of  dry  sodium  carbonate  and  one 
part  of  potassium  nitrate,  extracts  the  soluble  matter  with  warm 
water,  conducts  carbon  dioxide  into  the  filtrate  to  precipitate 
any  lead  which  may  be  present,  filters,  boils  the  insoluble  residue 
in  a  solution  of  sodium  carbonate,  filters  again,  and  boils  the 
residual  solid  matter  with  water  to  which  a  little  sodium  car- 
bonate has  been  added.    The  filtrates  and  wash  water  are  mixed, 
acidified  with  hydrochloric  acid,  warmed  to  expel  carbon  dioxide, 
and  repeatedly  evaporated  in  a  porcelain  dish  with  hydrochloric 
acid  in  order  to  expel  the  nitric  acid  and  to  render  insoluble  any 
silica  which  may  be  present.    The  residue  is  moistened  with  2  cc. 
of  hydrochloric  acid,  dissolved  in  water,  filtered  if  necessary,  and 
the  sulphuric  acid  precipitated  by  barium  chloride.     The  barium 
sulphate  obtained  in  this  way  is  frequently  not  quite  pure ;   it 
is  therefore  heated  repeatedly  in  the  platinum  crucible  in  which 
it  was  weighed  with  small  portions  of  weak  hydrochloric  acid, 
which  are  poured,  after  dilution,  through  a  small  filter.     The 
filtrate  is  treated  with  a  little  barium  chloride  and  evaporated 
nearly  to  dryness  on  the  water  bath.     The  residue  is  treated 

*  Zeitsch.  anal  Chem.,  19,  419.  t  Ibid.,  16,  335. 


SULPHUR  251 

with  water  and  filtered  through  the  small  paper,  which  is  after- 
wards burned,  the  ash  being  added  to  the  main  body  of  the 
barium  sulphate.  The  purified  material  is  usually  found  to 
weigh  a  little  less  than  the  original  precipitate. 
\-  (c)  C.  Meineke  *  and  O.  N.  Heidenreich  f  reduce  the  ferric 
salt  to  the  ferrous  condition  with  metallic  zinc,  filter,  wash  the 
undissolved  metal,  and  precipitate  the  sulphuric  acid  in  the 
filtrate  with  barium  chloride. 

(d)  F.  W.  Kiister  and  A.  Thiel  $  treat  the  cold  solution  with 
an  excess  of  dilute  ammonia,  heat  nearly  to  the  boiling  point, 
add  barium  chloride,  and  then  dissolve  the  precipitated  ferric 
hydroxide  with  hydrochloric  acid. 


The  solutions  of  the  sulphide  are  to  be  treated  by  methods 
c  and  d. 

1.  Method  c 

Dilute  one  of  the  solutions  in  an  Erlenmeyer  flask  to  a  vol- 
ume of  200  or  250  cc.,  add  granulated  zinc  —  or  filings  of  the 
metal  —  and  hydrochloric  acid ;  place  a  funnel  in  the  mouth  of 
the  flask  and  warm  upon  a  sand  or  asbestus  bath  until  the 
reduction  is  complete.  Filter  hot  —  through  a  paper  wet  with 
boiling  water  —  into  a  beaker,  wash  the  undissolved  metal  and 
the  filter,  and  precipitate  the  sulphuric  acid  with  the  usual  pre- 
cautions. 

2.  Method  d 

Dilute  the  remaining  solution  of  the  mineral  to  70  or  80  cc. 
and  precipitate  the  iron  with  a  moderate  excess  of  dilute  ammo- 
nia. Heat  nearly  to  the  boiling  point,  stirring  meantime,  and 
then  —  without  separation  of  the  ferric  hydroxide  by  filtration 

*  Zeitsch.  anal  Chem.,  38,  209,  351. 
t  Zeitsch.  anorg.  Chem.,  20,  233. 
t  Ibid.,  19,  97. 


252  QUANTITATIVE  EXERCISES 

— precipitate  the  sulphuric  acid  with  barium  chloride.  Dissolve 
the  ferric  hydroxide  with  dilute  hydrochloric  aci'd,  and  keep  the 
liquid  hot  for  two  hours.  After  cooling  to  the  temperature  of 
the  room,  pour  the  liquid  through  a  filter  and  digest  the  remain- 
ing barium  sulphate  for  half  an  hour  with  water  containing  a 
little  hydrochloric  acid.  Decant  through  the  filter ;  repeat  the 
digestion;  wash  several  times  with  boiling  water,  by  decanta- 
tion ;  and  then  bring  the  precipitate  upon  the  filter. 


EXERCISE  XXIII 

THE   DETERMINATION   OF    SULPHUR   IN   IRON  BY 
FRESENIUS'   METHOD 

There  are  required  for  this  experiment : 

1.  A  Kipp's  apparatus  for  the  generation  of  hydrogen. 

2.  Three  cylinders,  or  wash  bottles,  for  the  liquids  employed 
to  purify  the  hydrogen.     The  first,  i.e.  that  next  to  the  genera- 
tor, is  to  contain  a  solution  of  caustic  soda ;  the  second,  a  neu- 
tral or  slightly  alkaline  solution  (under  no  circumstances  an 
acid  one)  of  potassium  permanganate ;  and  the  third,  a  solution 
of  lead  hydroxide  in  caustic  soda.     The  last  is  made  by  adding 
an  excess  of  sodium  hydroxide  to  a  solution  of  lead  nitrate. 

3.  A  quarter-liter  flask  to  contain  the  hydrochloric  acid  which 
is  used  to  dissolve  the  iron.     This  is  supplied  with  a  doubly 
perforated  rubber  stopper,  through  which  pass  two  glass  tubes, 
both  bent  to  a  right  angle.     One  of  the  tubes  reaches  to  the 
bottom  of  the  flask,  and  is  covered  with  vaseline  where  it  passes 
through  the  stopper  in  order  that  it  may  be  easily  raised  and 
lowered.     The  other  —  the  one  which  is  connected  with  the 
washing  apparatus  —  reaches  only  about  halfway  to  the  bottom 
of  the  flask.     The  flask  is  to  be  filled  about  half  full  of  pure 
concentrated  hydrochloric  acid.  w 

4.  A  300-  or  400-cc.  flask  in  which  to  dissolve  the  iron. 
This,  like  the  flask  3,  is  supplied  with  a  doubly  perforated 


SULPHUR  253 

stopper  and  two  glass  tubes.  One  of  the  latter  is  long  —  reach- 
ing to  the  bottom  of  the  flask  —  and  is  bent  to  a  right  angle. 
This  is  connected  with  the  long  tube  in  3  by  a  piece  of  rubber 
tubing  100  or  150  mm.  in  length.  This  rubber  tube,  also  that 
by  which  3  is  connected  with  2,  is  provided  with  a  Mohr  pinch- 
cock.  The  second  glass  tube,  which  should  be  as  wide  as  prac- 
ticable, is  bent  to  less  than  a  right  angle  and  is  cut  off  even 
with  the  lower  end  of  the  stopper.  The  flask  is  to  be  placed 
on  a  tripod. 

5.  A  short  condenser  with  a  comparatively  wide  inner  tube. 
The  condenser  is  fixed  in  an  inclined  position  with  its  lower 
end  attached  to  the  tube  in  4,  which  is  bent  to  less  than  a 
right  angle. 

6.  Two  Peligot  tubes,  which  are  to  be  connected  with  each 
other  and  with  the  condenser.     In  these,  corks,  and  not  rubber 
stoppers,  must  be  used.     Both  tubes  are  to  be  partly  filled  with 
strong  hydrochloric  acid  in  which  from  3  to  5  cc.  of  bromine 
have  been  dissolved.     At  the  exit  end  they  are  to  be  connected 
with  a  flask  containing  sodium  hydroxide  to  which  hydrogen 
peroxide  has  been  added. 

Weigh  about  10  grams  of  cast-iron  borings  into  the  flask  4, 
and  add  a  few  cubic  centimeters  of  water.  Conduct  hydrogen 
through  the  apparatus — with  the  Peligot  tubes  detached — until 
the  air  in  it  has  been  displaced.  Attach  the  Peligot  tubes,  press 
down  the  long  tube  in  3,  and  let  the  hydrogen  force  a  little 
hydrochloric  acid  into  4.  Clamp  the  rubber  tube  between  3  and 
4,  and  gently  heat  the  flask  4.  When  the  reaction  becomes  too 
moderate,  introduce  more  acid  in  the  same  manner  as  before. 
When  the  solution  of  the  iron  has  been  completed,  raise  the 
long  tube  in  3,  and  again  pass  hydrogen  through  the  apparatus. 
Transfer  the  contents  of  the  Peligot  tubes  to  a  porcelain  dish, 
add  about  a  gram  of  pure  potassium  chloride,  and  evaporate  to 
dryness  at  a  moderate  temperature.  Dissolve  the  residue  in 
water,  add  a  little  hydrochloric  acid,  and  precipitate  the  sul- 
phuric acid  in  the  usual  manner. 


254 


QUANTITATIVE  EXERCISES 


The  insoluble  matter  in  the  flask  —  largely  carbon  —  will 
probably  contain  a  small  amount  of  sulphur,  and  this  must  also 
be  converted  into  sulphuric  acid  and  determined  with  that 
already  precipitated.  Filter  the  contents  of  the  flask  and  wash 
thoroughly,  first  with  a  little  dilute  hydrochloric  acid  and  then 
with  hot  water.  Dry  the  filter  and  brush  the  material  in  it  into 
a  mortar.  The  small  quantity  of  material 
which  cannot  be  separated  from  the  paper 
may  safely  be  neglected.  Add  10  grams 
of  dry  sodium  carbonate  and  5  grams  of 
.  potassium  nitrate,  and  grind  to  a  uniform 
mixture.  Transfer  the  contents  of  the 
mortar  to  a  large  platinum  crucible  and 
heat —  raising  the  temperature  quite  grad- 
ually and  only  to  the  fusing  point  of  the 
salts.  Dissolve  the  cooled  mass  in  water, 
filter  if  necessary,  and  evaporate  the  solu- 
tion in  porcelain  to  dryness  with  hydro- 
chloric acid.  Moisten  the  residue  with 
strong  hydrochloric  acid  and  again  evap- 
orate to  dryness,  repeating  the  treatment 
until  all  the  nitric  acid  has  been  expelled. 
Dissolve  the  residue  in  water,  adding  a 
few  drops  of  hydrochloric  acid,  filter,  and 
precipitate  the  sulphuric  acid  with  barium 
chloride.  Collect  both  portions  of  barium 
sulphate  on  the  same  filter. 

All  the  materials  used  in  the  experi- 
ment must  be  examined  for  sulphur ;  and,  if  reagents  free  from 
it  are  not  obtainable,  the  sulphur  in  them  must  be  determined. 
It  will  then  be  practicable  by  .working  with  weighed  or  meas- 
ured quantities  of  the  impure  reagents  to  correct  properly  the 
final  results. 

This  method  for  the  determination  of  sulphur  in  iron  can 
be  applied  with  advantage  to   all  those  sulphides  which  are 


FIG.  47 


SULPHUR  255 

decomposed  by  hydrochloric  acid  with  a  quantitative  conversion 
of  the  sulphur  into  hydrogen  sulphide. 

A  more  convenient  form  of  apparatus  is  shown  in  Fig.  47. 
The  tube  A  is  filled  with  beads  or  broken  glass,  and  the  bulb  c, 
with  the  solution  of  bromine  in  hydrochloric  acid.  The  iron,  or 
the  sulphide  in  which  the  sulphur  is  to  be  determined,  is  placed 
in  B.  The  air  in  B  is  expelled  by  hydrogen  which  is  purified 
in  the  manner  already  described,  the  gas  being  allowed  to 
escape  first  at  a  and  afterwards  through  b.  The  bulb  d  is  then 
filled  with  hydrochloric  acid  which  is  admitted  to  the  material 
below  as  occasion  may  require.  Previous  to  the  introduction 
of  the  acid,  however,  the  broken  glass  in  A  is  thoroughly  moist- 
ened with  the  bromine  solution  in  c.  Afterwards  the  solution 
is  allowed  to  flow  from  c  as  rapidly  as  may  be  necessary  in 
order  to  keep  the  glass 
well  colored  by  the  bro- 
mine. The  solution  con- 
taining the  sulphuric  acid, 
which  collects  at  the  bot- 

x  IG.  4o 

torn  of   the  apparatus,   is 

drawn  off  from  time  to  time  into  a  beaker  or  flask ;  and  when 
the  operation  is  complete,  the  broken  glass  and  the  connecting 
tube  b  are  rinsed  with  water  into  the  same  receptacle. 

Still  another  very  effective  form  of  apparatus  for  the  ab- 
sorption of  hydrogen  sulphide  and  other  gases  is  shown  in 
Fig.  48. 

Various  other  substances  besides  a  solution  of  bromine  in 
hydrochloric  acid  are  employed  to  absorb  or  to  oxidize  hydrogen 
sulphide ;  for  example,  solutions  of  lead  oxide  in  caustic  alkali ; 
of  cadmium  oxide  in  ammonia ;  of  silver  oxide  in  ammonia ;  of 
ammoniacal  hydrogen  peroxide;  and  of  potassium  permanga- 
nate. It  may  also  be  determined  iodometrically : 

L  +  H9S  =  S  +  2  HI. 


256  QUANTITATIVE  EXERCISES 


EXERCISE   XXIV 

THE   DETERMINATION   OF    SULPHUR   IN  ORGANIC 
COMPOUNDS 

I.   BY  LIEBIG'S  METHOD 

Place  in  a  silver  crucible  about  10  grams  of  potassium  hydrox- 
ide which  is  free  from  sulphur.  Fuse  (over  an  alcohol  lamp  if 
practicable),  and  continue  to  heat  until  the  liquid  becomes  tran- 
quil. Add  from  1  to  1.5  grams  of  potassium  nitrate  and  stir 
it  into  the  molten  hydroxide  with  a  silver  spatula.  Allow  the 
crucible  to  cool,  and  place  on  the  top  of  the  solidified  mixture 
a  weighed  quantity  of  any  non-volatile  organic  sulphur  com- 
pound which  contains  about  20  milligrams  of  sulphur.  Fuse 
the  contents  of  the  crucible,  and  continue  to  heat  until  the 
carbon  which  separates  is  all  oxidized  and  the  liquid  becomes 
transparent.  Dissolve  in  water,  and  evaporate  the  solution  in 
a  porcelain  dish  repeatedly  with  an  excess  of  hydrochloric  acid 
until  all  the  nitric  acid  has  been  expelled.  Dissolve  the  residue 
in  water  containing  a  little  hydrochloric  acid.  Dilute  to  about 
a  liter  and  allow  the  solution  to  stand  for  several  hours,  observ- 
ing whether  any  silver  chloride  separates.  If  it  becomes  cloudy, 
filter.  Evaporate  in  a  porcelain  or  platinum  dish  to  150  or 
200  cc.  and  precipitate  the  sulphuric  acid  in  the  usual  manner 
with  barium  chloride. 

If  gas  must  be  used,  since  it  always  contains  sulphur,  care 
must  be  taken  to  protect  the  contents  of  the  crucible  from  the 
products  of  combustion,  and  the  fusion  should  not  be  unneces- 
sarily prolonged.  It  is  to  be  remembered  in  this  connection 
that  much  of  the  sulphur  ordinarily  in  illuminating  gas  may 
be  removed  by  passing  the  gas  over  a  solution  of  lead  oxide  in 
sodium  hydroxide. 


SULPHUR  257 

II.   BY  CARIUS'  METHOD 

In  a  thick-walled  tube  of  moderately  soft  glass  —  closed  at 
one  end  and  having  a  length  of  450  or  500  mm.  and  an  internal 
diameter  of  15  to  18  mm.  —  place  a  weighed  quantity  of  any 
pure  organic  sulphur  compound  which  contains  from  15  to  20 
milligrams  of  sulphur.  Calculate  how  much  of  the  nitric  acid 
which  was  prepared  for  use  in  Exercise  XXII  will  be  required 
for  the  complete  oxidation  of  the  compound,  and  measure  about 
three  times  that  amount  into  a  narrow  glass  tube.  Place  the 
tube  containing  the  acid  in  the  larger  one,  taking  care  not  to  allow 
the  acid  and  the  substance  to  be  oxidized  to  come  in  contact. 
Close  the  outer  tube  with  great  care  as  directed  in  Exercise 
XIX,  II.  Heat  the  tube  also  as  there  directed. 

The  temperature  to  which  the  bath  must  be  raised  and  the 
length  of  time  which  it  will  be  necessary  to  heat  the  contents 
of  the  tube  will  depend  on  the  nature  of  the  substance.  If  it  is 
easily  oxidized,  two  hours'  heating  at  200°  will  suffice.  If  it 
is  attacked  with  difficulty  by  oxidizing  agents  —  like  some  of 
the  sulphur  derivatives  of  the  aromatic  hydrocarbons,  especially 
the  sulphonic  acids  —  it  will  be  necessary  to  heat  four  or  six 
hours  at  250°  or  260°. 

Remove  the  tube  from  the  bath  and  open  it  with  all  the  pre- 
cautions mentioned  in  Exercise  XIX,  II.  Transfer  the  liquid 
containing  the  sulphuric  acid  to  a  porcelain  dish,  and  add  about 
a  gram  of  pure  potassium  chloride.  Evaporate  with  hydrochloric 
acid  repeatedly  to  remove  nitric  acid,  and  determine  the  sulphuric 
acid  as  barium  sulphate. 

The  ratio  of  the  weight  of  the  nitric  acid,  in  grams,  to  the 
capacity  of  the  tube  in  cubic  centimeters  should  not  exceed  that 
of  1  to  12.  In  the  case  of  substances  requiring  a  high  tempera- 
ture for  their  decomposition,  e.g.  300°,  the  tube  may  be  heated 
for  a  time  at  a  lower  temperature,  e.g.  250°,  then  cooled  and 
opened  to  relieve  the  pressure.  It  may  then  be  resealed  and 
heated  with  safety  to  the  required  temperature. 


258  QUANTITATIVE  EXERCISES 

Liquid  substances  which  are  too  volatile  for  manipulation  in 
open  tubes  are  weighed  in  bulbs  blown  on  the  end  of  thin,  nearly 
capillary  tubes.  A  file  mark  made  near  the  bulb  will  enable  one 
to  break  off  the  stem  after  sealing  the  outer  tube. 


III.    BY  SAUER'S  METHOD 
For  Liquids 

The  apparatus  required  for  the  experiment  is  shown  in  Fig.  49. 
A  combustion  tube  850  mm.  long  is  narrowed  down  at  the 
point  a  to  a  diameter  of  about  5  mm.  The  rear  end  is  closed 
with  a  stopper  through  which  passes  one  limb  of  the  branching 
tube  b,  c.  The  front  end  is  closed  with  a  stopper  through  which 


FIG.  49 

pass  two  tubes.  One  of  these  is  cut  off  even  with 
the  smaller  end  of  the  stopper  and  serves  to  con- 
nect the  combustion  tube  with  the  Peligot  tube  e.  The  other, 
t?,  extends  into  the  larger  tube  to  the  constriction  a.  The  bend 
at  /  is  made  large  so  that  the  portion  from  d  to  /  may  lie 
outside  of  the  furnace.  If  desired,  the  tube  may  be  cut  near/ 
and  the  two  parts  joined  by  a  piece  of  rubber  tubing.  The 
absorption  apparatus  e  contains  a  solution  of  bromine  in  hydro- 
chloric acid,  and  is  connected  in  turn  with  another  apparatus  of 
the  same  kind  which  is  partly  filled  with  caustic  soda  to  which 
hydrogen  peroxide  has  been  added.  The  tubes  c  and  d  are  con- 
nected with  a  gasometer  containing  oxygen.  The  branch  b  is 
connected  with  a  Kipp's  carbon  dioxide  generator.  Both  gases 
are  dried  by  calcium  chloride  before  entering  the  combustion 
tube.  The  rubber  connections  of  the  tubes  5,  c,  and  d  are  pro- 
vided with  screw  pinchcocks  in  order  that  the  flow  of  the  gases 
may  be  regulated. 


SULPHUR 

In  a  bulb  blown  on  the  end  of  a  narrow,  thin  tube,  weigh  a 
quantity  of  any  pure  liquid  organic  sulphur  compound  which 
contains  from  15  to  20  milligrams  of  sulphur.  Make  a  file 
mark  near  the  bulb,  and  lay  the  bulb  in  a  porcelain  boat  with 
the  scratch  underneath  and  the  stem  extending  over  the  end  of 
the  boat.  Place  the  boat  in  the  combustion  tube  in  the  position 
indicated  in  the  figure.  Pass  carbon  dioxide  through  the  appa- 
ratus and  heat  the  combustion  tube  at  a  to  redness.  Remove 
the  rear  stopper,  and  break  the  tube  containing  the  liquid  by 
pressing  upon  it  with  a  glass  or  iron  rod  just  over  the  file  mark. 
Replace  the  stopper  quickly,  slow  down  the  current  of  car- 
bon dioxide,  admit  oxygen  through  d,  and  cautiously  warm  the 
material.  The  flow  of  carbon  dioxide  through  6,  and  of  oxygen 
through  c?,  also  the  heating  of  the  substance,  must  all  be  so 
regulated  that  the  combustion  —  which  will  consist  of  a  series 
of  flashes  —  will  take  place  in  the  immediate  vicinity  of  the  con- 
striction a.  If  the  flame  recedes  towards  the  boat,  the  flow  of 
carbon  dioxide  must  be  increased  or  the  temperature  must  be 
increased.  If  it  moves  too  far  in  the  other  direction,  the  flow 
of  oxygen  through  d  must  be  increased  or  the  temperature  of  the 
substance  lowered.  When  there  is  no  more  volatile  matter  to 
burn,  cut  off  the  carbon  dioxide,  admit  oxygen  through  <?,  and 
heat  the  tube  in  the  vicinity  of  the  boat  to  redness.  Fill  the 
apparatus  with  carbon  dioxide.  Transfer  the  contents  of  e  to 
a  porcelain  dish  and  rinse  into  the  same  all  parts  of  the  various 
tubes  which  have  been  in  contact  with  the  products  of  the  com- 
bustion. Add  potassium  chloride,  evaporate,  and  determine  the 
sulphuric  acid  as  barium  sulphate. 


Instead  of  a  solution  of  bromine  in  hydrochloric  acid,  one 
may  employ,  as  proposed  by  Burton,  a  standard  solution  of 
sodium  carbonate  to  absorb  the  sulphur  dioxide  and  neutralize 
the  sulphuric  acid,  and  determine  the  excess  of  the  alkali  volu- 
metrically  with  use  of  methyl  orange  as  the  indicator. 


260  QUANTITATIVE  EXERCISES 

When  the  method  of  Sauer  is  employed  on  substances  which, 
like  coke,  yield  no  volatile  products  at  high  temperatures,  the 
tube  d  is  omitted  and  the  material  is  burned  in  oxygen  which 
enters  the  combustion  tube  at  the  rear  end.  No  current  of  car- 
bon dioxide  is  then  required,  and  the  tube  need  not  be  constricted 
at  any  point. 

If  a  substance  yields  an  ash,  a  portion  of  the  sulphur  may 
remain  in  the  boat  as  an  insoluble  sulphate.  The  ash  must  then 
be  fused  with  an  alkaline  carbonate,  the  mass  treated  with  water, 
the  solution  filtered  and  evaporated  to  dryness  with  an  excess  of 
hydrochloric  acid  to  render  silica  insoluble,  and  the  residue 
digested  with  water  containing  a  little  hydrochloric  acid.  In 
the  filtrate,  the  sulphuric  acid  is  precipitated  with  barium 
chloride. 


CHAPTER  XI 
NITROGEN 


EXERCISE  XXV 
THE  DETERMINATION  OF  NITRIC   ACID 

I.     BY    THE    TlEMANN-SCHULZE    METHOD 

An  apparatus  like  that  represented  in  Fig.  50  is  required. 
The  flask  should  have  a  capacity  of  150  or  200  cc.  The  rubber 
stopper  must  be  a  soft  one  and  fit  the  flask  tightly.  The  glass 
tubes  which  pass  through  it  should  also  fit  very  closely.  The 
right-hand  tube,  through  which  the 
nitric  oxide  escapes  to  the  eudiom- 
eter, is  cut  off  flush  with  the  lower 
end  of  the  stopper  ;  while  the  left- 
hand  tube,  which  serves  for  the 
introduction  of  ferrous  chloride  and 
hydrochloric  acid,  extends  halfway 

down  the  neck  of  the  flask  and  is  drawn  out  to  a  fine  point. 
Both  of  the  tubes  are  cut  and  the  parts  rejoined  by  means  of 
thick- walled  but  very  flexible  rubber  tubing  of  the  black  variety. 
The  connectors  should  be  tied  with  waxed  shoemaker's  thread, 
and  each  is  to  be  provided  with  a  Mohr's  pinchcock.  The  end 
of  the  exit  tube,  where  it  passes  under  the  graduated  tube, 
should  be  covered  with  rubber. 

Test  the  apparatus  in  the  following  manner :  Fill  the  flask 
half  full  of  water.  Immerse  the  outlet  of  the  right-hand  tube 
in  half  a  liter,  and  that  of  the  left-hand  tube  in  a  few  cubic  cen- 
timeters of  water.  Boil  the  water  in  the  flask  down  to  a  volume 

261 


262  QUANTITATIVE  EXERCISES 

of  about  50  cc.  Close  the  left-hand  tube  for  a  few  minutes, 
then  open  it  and  close  the  right-hand  tube  for  a  time.  Close 
the  left-hand  tube  and  remove  the  lamp.  After  15  or  20  min- 
utes, open  the  right-hand  tube  and  allow  the  water  to  enter  the 
flask.  If  it  fills  the  flask  completely,  the  apparatus  is  sufficiently 
tight. 

Place  about  40  grams  of  iron  wire  in  a  flask  and  pour  over  it 
100  cc.  of  concentrated  hydrochloric  acid.  When  the  reaction 
is  nearly  over,  close  the  flask  loosely  with  a  cork. 

Put  half  a  liter  of  a  ten  per  cent  solution  of  caustic  soda  in  a 
porcelain  dish  and  boil  the  air  out  of  it. 

Weigh  about  one-tenth  of  a  gram  of  pure  potassium  nitrate 
into  the  flask  represented  in  the  figure,*  and  add  to  it  water 
until  the  flask  is  half  full.  Insert  the  stopper  and  boil  until 
the  volume  of  the  solution  has  been  reduced  to  40  or  50  cc. 
Close  the  left-hand  tube  and  immerse  the  outlet  of  the  right- 
hand  tube  in  the  solution  of  sodium  hydroxide.  After  the 
steam  has  been  conducted  for  a  time  into  the  caustic  soda, 
close  the  right-hand  tube  and  open  the  other  with  its  exit  end 
under  water.  When  the  volume  of  the  solution  in  the  flask  has 
been  reduced  to  about  10  cc.,  close  the  left-hand  tube  and  remove 
the  lamp. 

Fill  the  eudiometer  with  the  caustic  soda,  place  a  small  paper 
filter  on  the  convex  end  of  the  liquid  column,  and  invert  the 
tube  with  its  open  end  in  the  solution  of  alkali.  Tilt  the 
eudiometer  to  one  side  to  allow  the  paper,  also  the  bubble  of 
air  which  will  probably  be  under  it,  to  rise  to  the  top,  and  then 
fix  it  in  its  position  over  the  outlet  of  the  exit  tube.  Introduce 
into  the  flask,  through  the  left-hand  tube,  as  soon  as  the  dimin- 
ished pressure  will  permit  it,  20  cc.  of  the  saturated  solution  of 
ferrous  chloride,  and  afterwards  about  an  equal  volume  of  con- 
centrated hydrochloric  acid.  In  order  to  insure  the  removal  of 
all  the  ferrous  chloride  from  the  tube,  the  acid  is  divided  into 
two  portions  and  introduced  from  separate  vessels.  It  is  con- 
venient to  take  the  chloride  and  the  two  portions  of  acid  from 


NITROGEN  263 

test  tubes  on  which  strips  of  paper  have  been  pasted  to  indicate 
the  quantities  to  be  introduced.  The  lower  of  the  two  strips 
on  each  of  the  three  tubes  must,  of  course,  be  placed  some 
distance  above  the  bottom  in  order  to  avoid  the  danger  of 
exposing  the  end  of  the  tube  through  which  the  liquids  enter 
the  flask. 

Warm  the  flask  slowly,  opening  the  pinchcock  on  the  right- 
hand  tube  from  time  to  time  just  far  enough  to  ascertain  which 
way  the  liquid  in  the  tube  will  flow.  As  soon  as  the  pressure 
within  the  flask  is  sufficient  to  force  the  nitric  oxide  into  the 
eudiometer,  remove  the  pinchcock  and  continue  to  heat  until 
the  volume  of  the  gas  no  longer  increases. 

Place  a  small  cup,  having  a  handle,  under  the  open  end  of 
the  gas  tube,  and  transfer  it  to  a  long  glass  cylinder  which  is 
nearly  full  of  water.  The  cylinder  should  be  nearly  high 
enough  to  permit  the  complete  submersion  of  the  tube.  After 
fifteen  or  twenty  minutes,  raise  the  tube  with  a  pair  of  tongs 
until  the  water  within  and  without  is  on  the  same  level  and 
read  the  volume  of  the  gas.  Submerge  the  tube  for  a  short 
time,  then  raise  it  and  read  again.  The  second  reading  can  be 
made  more  quickly  than  the  first,  and  is  necessary  because  of 
the  effect  of  the  evaporation  from  the  wet  glass  upon  the  vol- 
ume of  the  gas.  Note  the  temperature  of  the  water  in  the 
cylinder  and  the  height  of  the  barometer.  Correct  the  volume 
of  the  gas  to  standard  conditions  of  temperature  and  pressure 
and  also  for  the  water  vapor  which  it  contains.  One  cubic 
centimeter  of  nitric  oxide  weighs  under  standard  conditions 
1.34192  milligrams. 

This  method  is  sufficiently  accurate  for  ordinary  purposes. 
There  are,  however,  several  sources  of  error  in  it:  The  hydro- 
chloric acid  contains  air;  the  nitric  oxide  is  somewhat  soluble 
in  caustic  soda,  and  more  soluble  in  water;  and,  finally,  the 
free  oxygen  in  the  water  unites  with  some  of  the  nitric  oxide, 
converting  it  into  NO2,  and  this  is  decomposed  by  the  water 
with  the  formation  of  nitrous  and  nitric  acids. 


264  QUANTITATIVE  EXERCISES 

The  reaction  which  takes  place  when  nitric  acid,  ferrous 
chloride,  and  hydrochloric  acid  are  brought  together  may  be 
represented  by  the  following  equation : 

HN03  +  3  FeCl2  +  3  HC1  =  2  H2O  -f  3  FeCl3  +  NO. 

It  is  not  influenced  by  the  presence  of  organic  matter,  and  for 
that  reason  is  usually  employed  for  the  determination  of  nitric 
and  nitrous  acids  in  water  analysis. 

There  are  various  methods,  besides  the  measurement  of  its 
volume,  by  which  the  nitric  oxide  may  be  determined : 

1.  Pelouze,  who  was  the  first  to  employ  the  reduction  of 
nitric  acid  by  ferrous  chloride  for  quantitative  purposes,  added 
to  the  acid  a  known  quantity  of  the  ferrous  salt  and  titrated 
the  excess  with  potassium  permanganate.     But  owing  to  the 
reconversion  of  the  nitric  oxide  into  nitric  and  nitrous  acids  by 
the  oxygen  of  the  air  and  the  water  vapor  within  the  flask,  to 
the  difficulty  of  wholly  expelling  the  nitric  oxide,  and  to  the 
loss  of  nitric  acid  by  evaporation,  the  method  in  its  original 
form  has  been  abandoned.    It  has,  however,  been  so  modified  by 
Fresenius  (Quantitative  Analyse,  1,  520)  as  to  give  excellent  re- 
sults.   The  modification  consists  in  the  expulsion  of  the  air  and 
the  nitric  oxide  from  the  vessel  in  which  the  nitric  acid  is  reduced 
by  a  current  of  hydrogen  or  of  carbon  dioxide,  and  in  the  titration 
of  the  excess  of  the  ferrous  chloride  by  potassium  bichromate 
or  of  the  ferric  chloride  by  stannous  chloride. 

2.  Another  excellent  method  is  that  proposed  by  Schloesing, 
and  variously  modified  in  its  details  by  others,  according  to 
which  the  nitric  oxide  is  converted  into  nitric  acid  and  the  latter 
determined  by  means  of  a  standard  solution  of  alkali.    The  reac- 
tion may  be  represented  as  follows : 

2  NO  +  H2O  +  3  O  =  2  HNO3. 

3.  The  nitric  oxide  may  also  be  estimated  by  determining  its 
reducing  action  on  a  standard  solution  of  potassium  perman- 
ganate. 


NITROGEN  265 


II.    BY  CRUM'S  METHOD 

Turn  the  three-way  stopcock  of  the  Lunge  nitrometer,  repre- 
sented in  Fig.  51,  so  that  the  graduated  portion  of  the  tube  is 
in  communication  with  the  funnel  above.     Raise  the  reservoir 
tube  and  pour  mercury  into  it  until  the  metal  runs  into  the 
funnel.     Turn  the  stopcock  so  as  to  allow  the  mercury  in  the 
funnel  to  flow  out  at  the  side  and  then  turn  it  again      ^ 
so  as  to  close  the  funnel.     Place  in  the  bottom  of  the 
funnel  about  100  milligrams  of  pure  potassium  nitrate. 
Dissolve  it  in  the  least  possible  quantity  of  water, 
using  a  platinum  wire  as  a  stirring  rod.     Lower  the 
reservoir  tube  so  as  to  produce  a  slightly  diminished 
pressure.     Open  the  stopcock  cautiously  and  let  the 
solution  of  nitrate  run  slowly  into  the  tube  below, 
closing  it  again  before  the  air  enters  the  stopcock. 
Pour    about  1   cc.   of   pure    concentrated  sulphuric 
acid  into  the  funnel,  and  with  the  platinum  wire  rub 
it  over  the  glass  wherever  it  is  wet  with  the  aqueous 
solution  of  the  nitrate.     Let  the  sulphuric  acid  into 
the  tube  below  and  repeat  the  washing  of  the  funnel 
with  1-cc.  portions    of  sulphuric  acid  until  from 
6  to  8  cc.  have  been  introduced  into  the  graduated 
tube. 

Shake  the  tube  vigorously  up  and  down  for  five 
minutes,  taking  care,  however,  not  to  allow  the 
acid  to  come  in  contact  with  the  rubber  tube.  FlG- 
When  the  apparatus  has  cooled  to  the  temperature  of  the  room, 
agitate  again  in  order  to  ascertain  whether  the  reaction  is  fin- 
ished. Raise  or  lower  the  reservoir  until  the  mercury  in  it 
stands  above  that  in  the  measuring  tube  by  a  distance  equal  to 
one-seventh  the  length  of  the  sulphuric  acid  column,  and  read 
the  volume  of  the  gas.  Note  the  temperature  and  the  height 
of  the  barometer.  To  determine  whether  the  pressure  under 
which  the  gas  was  measured  is  really  equal  to  that  of  the  air, 


266  QUANTITATIVE  EXERCISES 

open  the  stopcock  and  observe  whether  the  mercury  in  the  grad- 
uated tube  rises  or  falls  and  to  what  extent.  If  any  change  in 
the  level  of  the  mercury  takes  place,  twice  the  amount  of  that 
change,  measured  in  millimeters,  must  be  added  to  or  subtracted 
from  the  height  of  the  barometer  when  the  volume  of  the  gas  is 
reduced  to  standard  pressure.  If  the  mercury  rises  on  opening 
the  stopcock,  the  correction  is  to  be  added ;  if  the  mercury  falls, 
it  is  to  be  subtracted. 

The  reduction  of  nitric  acid  by  mercury  and  sulphuric  acid 
may  be  represented  as  follows : 

2  HN03  +  6  Hg  +  3  H2S04  =  4  H2O  +  3  Hg2SO4  +  2  NO. 

Nitrous  acid  and  nitrogen  dioxide  are  also  reduced  to  nitric 
oxide  by  mercury  and  sulphuric  acid.  The  reaction  is  not 
affected  by  the  presence  of  organic  matter,  and  it  may  there- 
fore be  employed  in  water  analysis.  The  method  is  especially 
adapted  to  the  determination  of  oxides  of  nitrogen  in  sulphuric 
acid. 

There  are  various  other  methods  of  determining  nitric  acid. 

One  of  them,  Wagner's,  like  those  already  given,  involves  the 
reduction  of  the  acid  to  nitric  oxide.  It  is  based  on  the  reaction 
which  takes  place  when  a  nitrate  is  heated  with  chromium  oxide 
in  the  presence  of  an  alkaline  carbonate : 

2  KN03  +  Cr203  =  K2Cr2O7  +  2  NO. 

Either  or  both  of  the  products  of  the  reaction  may  be  determined. 

Another  method  which  is  sometimes  employed  in  water 
analysis  is  founded  on  the  fact  that  nitric  acid  destroys  the 
color  of  indigo  solution. 

There  are  also  several  methods  based  on  the  reduction  of  nitric 
acid  to  ammonia.  This  can  be  effected  in  a  variety  of  ways,  but 
most  readily  by  the  action  of  a  metal  or  a  pair  of  metals  on  an 
alkaline  solution  of  a  nitrate.  The  metals  usually  employed  are 
aluminium,  platinum  and  zinc,  iron  and  zinc,  zinc  and  copper. 


NITROGEN  267 


EXERCISE   XXVI 

COLORIMETRIC  DETERMINATION  OF  NITROUS  ACID  BY 
THE  METHOD  OF  GRIESS,  PREUSSE,  AND  TIEMANN 

The  reagents  required  for  this  experiment  are : 

1.  A  dilute  solution  of  metadiamidobenzene,  which  is  made 
by  dissolving  1.25  grams  of  the  base,  or  an  equivalent  amount 
of  a  salt,  in  water,  slightly  acidifying  with  dilute  sulphuric  acid, 
and  diluting  with  water  to  250  cc.     If  the  solution  is  colored,  it 
must  be  filtered  through  charcoal  or  agitated  with  a  small  amount 
of  the  same  and  then  filtered. 

2.  Dilute  sulphuric  acid  prepared  by  dissolving  one  volume  of 
the  strong  acid,  free  from  nitrous  acid,  in  two  volumes  of  water. 

3.  A  standard  solution  of  potassium  nitrite  containing  in  each 
cubic  centimeter  0.01  milligram  of  N2O3.     This  is  obtained  by 
dissolving  0.4047  gram  of  pure  and  dry  silver  nitrite  in  hot 
water  and  adding  to  the  solution  an  excess  of  potassium  chloride. 
After  cooling,  the  solution  is  diluted  to  a  liter,  and,  when  the 
silver  chloride  has  subsided,  100  cc.  of  it  are  withdrawn  and 
again  diluted  to  a  liter. 

The  silver  nitrite  is  prepared  by  mixing  a  warm  concentrated 
solution  of  10  parts  of  potassium  nitrite  with  a  warm  concen- 
trated solution  of  16  parts  of  silver  nitrate.  The  precipitate, 
when  cold,  is  washed  on  a  filter  with  cold  water,  and  dried,  first 
by  drawing  air  with  the  aid  of  the  filter  pump  through  the 
funnel  (tightly  covered  with  a  filter  paper),  and  then  in  a  desic- 
cator protected  from  the  light.  Or  it  may  be  quickly  dried  over 
a  water  bath.  In  the  latter  case,  however,  it  is  apt  to  suffer  a 
slight  decomposition. 

There  are  also  required  four  uniform  graduated  glass  cylin- 
ders such  as  are  employed  in  the  colorimetric  determination  of 
ammonia  and  known  under  the  name  of  Nessler  tubes,  and  two 
burettes,  one  for  the  sulphuric  acid  and  the  other  for  the  meta- 
diamidobenzene. 


268  QUANTITATIVE   EXERCISES 

For  the  experiment  a  specimen  of  well  water  containing 
nitrous  acid,  or  a  portion  of  the  standard  solution  of  nitrite 
diluted  to  a  known  degree  by  another  person  than  the  experi- 
menter may  be  used. 

Place  a  measured  portion  of  the  solution  to  be  tested  in  one 
of  the  cylinders,  add  1  cc.  of  the  sulphuric  acid  and  1  cc.  of  the 
metadiamidobenzene  solution,  fill  to  the  mark  on  the  cylinder 
with  water,  stir  with  a  glass  rod,  and  watch  the  color  which  is 
developed.  If  it  contains  even  the  slightest  shade  of  red,  the 
solution  is  too  concentrated  and  measured  portions  of  it  must  be 
diluted  with  measured  quantities  of  water  until  only  light  shades 
of  yellow  are  produced  by  the  metadiamidobenzene.  The  solu- 
tion is  sufficiently  concentrated  when  a  distinct  color  appears 
after  a  lapse  of  one  or  two  minutes. 

Place  100  cc.  of  the  properly  diluted  solution  in  one  of  the 
cylinders.  Into  the  others  measure  0.1-,  1.0-,  and  2.0-cc.  portions 
of  the  standard  solution  of  nitrite  and  fill  with  water  to  the  100-cc. 
marks.  Add  to  each  of  the  four  cylinders  1  cc.  of  sulphuric  acid 
and  1  cc.  of  the  metadiamidobenzene.  Stir  the  contents  of  each 
cylinder  with  a  separate  glass  rod.  Place  in  tubes  of  white  paper 
—  made  by  wrapping  strips  of  filter  paper  about  the  cylinders  and 
fastening  them  with  pins  —  and  set  them  on  a  white  surface. 

After  about  20  minutes  compare  the  color  in  the  solution  to 
be  tested  with  that  of  the  standards.  If,  as  will  probably  be 
the  case,  the  color  falls  between  those  of  two  of  the  standards, 
empty  all  the  tubes  and  repeat  the  experiment  with  other  stand- 
ards containing  quantities  of  nitrite  ranging  between  the  limits 
previously  established.  Proceed  in  this  manner  until  a  standard 
is  obtained  in  which  the  color  is  identical  with  that  of  the  solu- 
tion to  be  tested. 

The  colors  are  compared  by  looking  downward  through  the 
tubes.  The  depth  of  the  color  increases  with  time  ;  the  metadi- 
amidobenzene must  therefore  be  added  to  the  solution  to  be  tested 
and  to  the  standards  as  nearly  simultaneously  as  possible.  Much 
time  may  be  saved  by  using  graduated  cylinders  with  side  tubes 


NITROGEN  269 

near  the  bottom  which  are  furnished  with  stopcocks  —  so-called 
"  Hehner  cylinders."  Only  two  of  these  are  required  for  a  deter- 
mination, since  the  darker  liquid  can  be  drawn  off  until  the  colors 
—  on  looking  through  them  in  a  downward  direction  —  appear 
to  be  identical.  The  concentration  of  the  solutions  so  compared 
is  inversely  proportional  to  their  vertical  height. 

When  two  solutions  are  compared  with  respect  to  the  intensity 
of  their  color,  care  should  be  taken  to  bring  both  of  them  into 
the  same  relation  to  the  light.  If  the  shades  of  color  in  two 
tubes  appear  to  be  identical,  the  positions  of  the  tubes  should 
be  exchanged  and  the  shades  again  compared. 

The  colored  compound  which  is  produced  when  nitrous  acid  and 
metadiamidobenzene  are  brought  together  is  triamidoazobenzene : 

2  C6H4(NH2)2  +  HN02  =  C12H13N6  +  2  H2O. 

The  reaction  is  so  delicate  that  one  can  determine  by  means 
of  it  0.03  of  a  milligram  of  N2O3  in  a  liter  of  water.  The  eye 
distinguishes  most  accurately  the  shades  of  color  which  are 
developed  in  solutions  containing  from  0.03  to  0.3  of  a  milli- 
gram of  N2O3  per  liter.  The  colorimetric  method  is  mainly 
employed  for  the  determination  of  nitrous  acid  in  excessively 
dilute  solutions,  as  in  contaminated  well  or  spring  waters.  For 
more  concentrated  solutions  the  permanganate  method  is  used. 
If  colored  waters  are  to  be  examined  by  this  process,  they  must 
first  be  decolorized.  For  this  purpose,  200  cc.  of  a  hard  water 
are  treated  with  3  cc.  of  a  sodium  carbonate  solution  (1 :  3)  and 
0.5  cc.  of  caustic  soda  (1:2);  while  soft  waters  are  treated  with 
a  few  drops  of  an  alum  solution.  If  this  treatment  fails  to 
precipitate  the  coloring  matter,  the  method  cannot  be  used. 

Another  process  for  the  colorimetric  determination  of  nitrous 
acid  in  very  dilute  solutions  is  that  known  as  Trommsdorf's. 
It  is  based  on  the  reaction  which  takes  place  when  zinc  iodide 
and  nitrous  acid  are  brought  together.  The  iodine  which  is 
liberated  —  and  indirectly  the  nitrous  acid  —  is  estimated  by 
the  blue  color  imparted  to  starch  paste. 


270  QUANTITATIVE  EXERCISES 

EXERCISE  XXVII 
COLORIMETRIC  DETERMINATION  OF  AMMONIA 

THE  DETERMINATION  OF  "FREE"  AND  "ALBUMINOID"  AMMONIA 

IN  WATER  BY  THE  METHOD  OF  WANKLYN, 

CHAPMAN,  AND  SMITH 

The  following  reagents  are  required : 

1.  A  large  supply  of  water  which  is  free  from  ammonia.     To 
obtain  this,  the  ordinary  distilled  water  is  acidified  with  sulphuric 
acid  and  redistilled.     The  first  portion  of  the  distillate  must  be 
rejected  because  it  contains  the  ammonium  salts  which  have 
collected  on  the  glass  within  the  condenser  and  the  receiver. 
Water  which  has  been  purified  in  this  manner  must  be  used  in 
making  up  all  the  reagents  and  in  all  operations  in  which  the 
use  of  water  containing  ammonia  can  affect  the  results  of  a 
determination. 

2.  A  solution  of  Nessler's  reagent.     To  prepare  this,  dissolve 
8.75  grams  of  potassium  iodide  and  3.25  grams  of  mercuric 
chloride  in  200  cc.  of  boiling  water.     When  a  clear  solution  is 
obtained,  add  drop  by  drop  a  cold  saturated  solution  of  mercuric 
chloride  until  a  slight  permanent  precipitate  of  mercuric  iodide 
appears.    Dissolve  in  the  solution  40  grams  of  potassium  hydrox- 
ide or  30  grams  of  sodium  hydroxide.     When  cold,  add  a  little 
more  of  the  mercuric  chloride  and  dilute  to  250  cc.     Allow  the 
solid  matter  to  subside  and  then  pour  the  clear  liquid  into  a 
bottle  having  a  good  stopper.     The  reagent  should  be  used 
from  a  smaller  bottle  which  is  filled  from  the  larger  supply  as 
occasion  requires.     The  solution  should  have  a  light  yellow 
color.     If  it  is  colorless,  more  mercuric  chloride  must  be  added. 
Its  sensitiveness  should  be  tested  from  time  to  time  with  very 
dilute  solutions  of  ammonia. 

3.  Two  standard  solutions  of  ammonium  chloride,  one  con- 
taining 1   milligram   of  NH3  in  each   cubic    centimeter,    and 
the  other  one-hundredth  as  strong.     The  first  is  obtained  by 


NITROGEN  271 

dissolving  3.1370  grams  of  pure  ammonium  chloride  in  water 
and  diluting  to  a  liter;  and  the  second,  by  diluting  10  cc.  of 
the  first  solution  to  a  liter. 

4.  A  solution  of  potassium  hydroxide  and  potassium  perman- 
ganate.    To  prepare  this,  dissolve  100  grams  of  the  former  and 
4  grams  of  the  latter  in  half  a  liter  of  water  and  distill  off  1 25  cc. 
Dilute  the  residue  to  half  a  liter  with  water  free  from  ammonia. 

5.  A  boiled  saturated  solution  of  sodium  carbonate.    For  the 
practice  determination,  a  specimen  of  a  water  known  to  be  impure 
or  one  which  has  been  rendered  impure  by  the  addition  of  sewage 
should  be  employed. 

Place  500  cc.  of  the  water  to  be  tested  in  a  well-cleansed 
tubulated  liter  retort.  If  the  water  is  soft,  or  if  it  has  an  acid 
reaction,  add  10  cc.  of  the  sodium  carbonate  solution.  Attach 
the  retort  to  a  clean  condenser  and  distill  over  into  a  measuring 
flask  200  cc.  of  water.  Replace  the  receiver  with  a  150-cc. 
measuring  flask.  Introduce  50  cc.  of  the  alkaline  permanga- 
nate solution  into  the  retort  and  continue  the  distillation  until 
the  receiver  is  filled  to  the  mark. 

It  remains  to  determine  colorimetrically  the  ammonia  in  the 
two  distillates.  The  first  contains  the  "  free  "  ammonia,  or  that 
which  was  already  formed  in  the  water;  and  the  second,  the 
"albuminoid"  ammonia,  or  that  which  was  produced  by  the  action 
of  the  alkaline  permanganate  on  the  nitrogenous  organic  matter. 

To  a  10-cc.  portion  of  the  first  distillate,  in  a  narrow  test 
tube,  add  1  cc.  of  the  Nessler  reagent.  Stir  with  a  glass  rod 
and  observe  the  color  which  is  developed.  If,  after  waiting  for 
a  few  minutes,  the  color  is  found  to  be  only  a  light  shade  of 
pure  yellow,  the  solution  is  not  too  concentrated.  If,  on  the 
other  hand,  red  is  detected  in  the  color,  measured  portions  of 
the  distillate  must  be  diluted  with  measured  quantities  of  water 
until  a  solution  is  obtained  in  which  the  reagent  will  produce 
no  red  whatever.  Determine  the  ammonia  exactly  as  nitrous 
acid  was  determined  in  the  preceding  experiment,  using  2  cc.  of 
the  Nessler  reagent  in  each  cylinder. 


272  QUANTITATIVE  EXERCISES 

Proceed  with  the  distillate  containing  the  "  albuminoid " 
ammonia  in  precisely  the  same  manner. 

The  results  should  be  stated  in  milligrams  of  ammonia  per 
liter  of  the  original  water,  or  parts  per  million. 

Regarding  the  significance  of  so-called  "  free "  and  "  albu- 
minoid "  ammonia  in  drinking  water,  see  Wanklyn's  Water 
Analysis. 

EXERCISE  XXVIII 

THE   DETERMINATION   OF  NITROGEN   IN   ORGANIC 
COMPOUNDS 

I.   BY  KJELDAHL'S  METHOD 

The  following  reagents  and  apparatus  will  be  required  for 
one  or  another  of  the  various  modifications  of  the  method : 

1.  Half  a  liter  of  pure  concentrated  sulphuric  acid,  to  which 
100  grams  of  phosphorus  pentoxide  have  been  added. 

2.  Metallic  mercury  or  the  oxide  of  mercury.     The  latter  is 
prepared  by  pouring  a  hot  solution  of  mercuric  chloride  into  a 
hot  solution  of  potassium  hydroxide.     The  commercial  oxide 
usually  contains  nitrogen  and  cannot  be  used. 

3.  Finely  ground  potassium  permanganate. 

4.  A  nearly  saturated  solution  of  caustic  soda. 

5.  A  solution  of  potassium  sulphide  made  by  dissolving  40 
grams  of  the  commercial  sulphide  and  diluting  to  a  liter ;  or  by 
dissolving  an  equivalent  quantity  of  the  hydroxide  in  water, 
dividing  the  solution  into  two  equal  parts,  saturating  one  of 
these  with  hydrogen  sulphide  and  adding  the  other  to  it.    ,The 
strength  of  the  solution  should  be  determined  by  titrating  into 
a  solution  containing   a  known  weight  of  mercury  until  the 
liquid  blackens  a  paper  moistened  with  lead  acetate. 

6.  Granulated  zinc,  also  zinc  dust. 

7.  Phenolsulphuric    acid  made  by  dissolving  50   grams   of 
phenol  in  enough  concentrated  sulphuric  acid  to  make  100  cc. 
of  the  solution. 


NITROGEN  273 

8.  Pure  sodium  thiosulphate. 

9.  A  one-tenth  normal  solution  of  ammonia. 

10.  A  one-fifth  normal  solution  of  sulphuric  acid. 

11.  A  pear-shaped  flask  with  long  neck,  known  as  a  Kjeldahl 
digestion  flask. 

12.  A  500-  or  600-cc.  flask  from  which  to  distill  the  ammonia. 

13.  A  Peligot  tube  in  which  to  absorb  the  ammonia. 

14.  A  condenser  with  an  inner  tube  of  block  tin. 

15.  A  glass  tube  from  150  to  200  mm.  in  length,  having  a 
diameter  about  equal  to  that  of  the  tin  tube  and  a  bulb  in  the 
middle  from  30  to  35  mm.  wide.     One  end  passes  through  a 
stopper  which  fits  the  distillation  flask,  while  the  other  end  is 
attached  to  the  tin  tube  of  the  condenser  with  a  piece  of  rubber 
tubing. 

16.  A  sifter  for  the  potassium  permanganate.     This  is  made 
by  slightly  enlarging  the  end  of  a  glass  tube  of  suitable  size 
and  binding  over  it  a  piece  of  very  fine  wire  gauze. 

a.    By  WilfartJis  Modification  of  KjeldahVs  Method 
(Not  applicable  to  nitro-compounds  or  to  nitrates) 

Weigh  from  150  to  175  milligrams  of  pure  uric  acid  into  the 
digestion  flask.  Add  0.7  gram  of  the  mercuric  oxide  or  0.6 
gram  of  mercury.  Measure  in  20  cc.  of  the  sulphuric  acid  to 
which  phosphorus  pentoxide  has  been  added.  Fasten  the  flask 
in  an  inclined  position  over  a  wire  gauze  (under  a  hood),  and 
heat  with  a  Bunsen  burner,  gently  at  first,  but  afterwards  for 
an  hour  and  a  half  to  the  boiling  point  of  the  liquid. 

In  the  case  of  most  organic  compounds  the  acid  blackens  in 
consequence  of  the  separation  of  carbon,  and  then  becomes 
lighter  in  color  as  the  oxidation  progresses.  The  black  gives 
way  by  degrees  to  a  dark  brown,  a  light  brown,  and  a  light  yellow 
color,  and  finally  the  liquid  becomes  colorless.  The  disappear- 
ance of  all  color  from  the  acid  is  usually  regarded  as  an  indica- 
tion of  the  complete  conversion  of  the  nitrogen  into  ammonia; 


274  QUANTITATIVE  EXERCISES 

but  in  the  case  of  uric  acid,  either  the  acid  does  not  blacken 
at  all  or  it  becomes  colorless  before  the  reaction  is  finished. 
When  the  light  yellow  stage  is  reached,  the  oxidation  is  usually 
assisted  by  sifting  in  a  little  of  the  pulverized  permanganate, 
though  in  most  cases  such  assistance  is  unnecessary. 

Place  the  flask  in  a  vertical  position  and  introduce  minute 
quantities  of  the  permanganate  by  tapping  the  sifter  on  the  side 
or  top  until  the  liquid  assumes  a  green  or  purplish  color.  Place 
about  200  cc.  of  water  in  the  distilling  flask  and  pour  the  con- 
tents of  the  digestion  flask  slowly  into  it.  Wash  the  latter  and 
add  the  washings  to  the  liquid  in  the  former.  Measure  50  cc. 
of  the  standard  acid  into  the  absorption  tube  and  attach  it  to 
the  condenser.  Having  made  ready  to  close  the  distillation 
flask  and  to  connect  it  with  the  condenser,  cautiously  neu- 
tralize the  sulphuric  acid  with  the  strong  alkali.  When  it  is 
believed  that  the  neutral  point  has  been  nearly  reached,  add  at 
once  a  considerable  excess  of  the  alkali,  also  about  one  and  a 
half  times  the  quantity  of  the  potassium  sulphide  which  is 
required  to  precipitate  the  mercury.  Throw  in  a  few  pieces  of 
granulated  zinc  and  quickly  close  the  flask.  Distill  off  from 
150  to  200  cc.  of  the  liquid  and  determine  the  excess  of  acid  in 
the  Peligot  tube  by  means  of  the  standard  solution  of  ammonia. 

To  ascertain  whether  any  portion  of  the  ammonia  has  been 
derived  from  the  reagents,  make  a  blank  determination,  using 
half  a  gram  of  pure  sugar  in  the  place  of  the  uric  acid. 

b.   Gunning's  Modification  of  KjeldahVs  Method 
(Not  applicable  to  nitro-compounds  or  to  nitrates) 

Weigh  two  5-cc.  portions  of  well-mixed  milk  into  digestion 
flasks  and  evaporate  to  dryness  in  a  water  or  air  bath.  Treat 
each  of  the  residues  with  10  grams  of  powdered  potassium  sul- 
phate and  20  cc.  of  concentrated  sulphuric  acid.  Place  funnels 
in  the  necks  of  the  flasks  and  cover  them  with  watch  glasses. 
Heat  cautiously  until  frothing  ceases,  and  then  more  strongly 


NITROGEN  275 

until  the  acid  becomes  colorless.  Complete  the  determination 
as  directed  under  a,  omitting  the  addition  of  potassium  per- 
manganate and  sulphide.  A  blank  determination  should  be 
made  in  order  to  ascertain  to  what  extent  the  results  are  to 
corrected  for  ammonia  in  the  reagents. 

c.  Jodlbauer's  Modification  of  Kjeldahl's  Method 

(Applicable  to  nitro-compounds  and  nitrates) 

Weigh  from  300  to  500  milligrams  of  pure  potassium  nitrate 
into  a  digestion  flask.  Add  to  it  2.5  cc.  of  the  phenolsulphuric 
acid  and  20  cc.  of  the  sulphuric  acid  which  has  been  treated  with 
phosphorus  pentoxide;  also  from  2  to  3  grams  of  zinc  dust  and 
five  drops  of  the  ordinary  platinum  chloride  solution.  Heat  until 
the  acid  becomes  colorless,  and  then  proceed  as  directed  under  b. 

d.  Foerster's  Modification  of  KjeldahVs  Method 

(Applicable  to  nitro-compounds  and  nitrates) 

Treat  from  300  to  500  milligrams  of  potassium  nitrate  with 
a  six  per  cent  solution  of  phenol  in  strong  pure  sulphuric  acid, 
adding  3  cc.  for  each  tenth  of  a  gram  of  the  nitrate.  Shake  the 
flask  lightly,  without  heating,  until  the  nitrate  is  dissolved, 
and  then  introduce  at  one  time  from  1  to  2  grams  of  sodium 
thiosulphate.  When  the  reaction  which  follows. is  finished, 
measure  in  a  quantity  of  the  strong  sulphuric  acid  equal  to 
two-thirds  the  volume  of  phenolsulphuric  acid  which  was  first 
added.  Pour  in  half  a  gram  of  mercury  and  heat  until  the 
contents  of  the  flask  become  colorless.  Finish  the  determina- 
tion as  directed  under  a. 

Instead  of  phenolsulphuric  acid,  a  solution  of  salicylic  acid 
in  sulphuric  acid  (1:30)  may  be  used. 

Gunning's  method  is  also  modified  so  as  to  apply  to  nitrates. 
The  material  is  treated  in  the  digestion  flask  with  30  cc.  of  sul- 
phuric acid  containing  1  gram  of  salicylic  acid.  The  contents 


276  QUANTITATIVE  EXERCISES 

are  thoroughly  mixed  and  frequently  shaken  for  a  period  of  ten 
minutes.  Five  grams  of  sodium  thiosulphate  and  10  grams 
of  potassium  sulphate  are  added.  The  mixture  is  heated 
gently  until  frothing  ceases,  and  then  more  strongly  until  it 
is  nearly  colorless.  The  determination  is  finished  as  directed 
under  b. 

The  object  in  using  phenol  and  salicylic  acid  in  the  case  of 
nitrates  is  to  convert  the  nitric  acid  into  nitro-derivatives  of 
these  substances,  and  the  reducing  agents  —  zinc  and  sodium 
thiosulphate  —  are  employed  to  convert  the  nitro-derivatives 
into  amido-compounds  which  are  decomposed  by  concentrated 
sulphuric  acid  with  conversion  of  all  of  the  nitrogen  into 
ammonia. 

.II.   BY  DUMAS'  METHOD 

Select  a  piece  of  hard  glass  tubing  having  an  internal  diam- 
eter of  15  or  16  mm.  and  a  length  greater  than  that  of  the  com- 
bustion furnace  by  about  150  mm.  Smooth  the  rough  ends  in 
the  flame  of  the  blast  lamp,  and  fit  each  with  a  perforated  stop- 
per through  which  a  small  glass  tube  has  been  passed.  At  a 
point  within  the  tube  about  one-third  of  its  length  from  one 
end,  place  a  short  roll  of  copper  wire  gauze  to  serve  as  a  parti- 
tion. Fill  the  longer  space  to  within  100  mm.  of  the  end  of 
the  tube  with  coarse  copper  oxide  and  confine  it  in  its  place  by 
inserting  a  short  roll  of  copper  wire  gauze.  Fill  the  shorter 
space  in  the  same  manner  with  fine  copper  oxide.  Insert  the 
stoppers  and  heat  the  tube  in  the  furnace,  raising  the  tempera- 
ture gradually  to  a  full  red  heat.  Pass  a  slow  current  of  oxy- 
gen through  the  tube  for  an  hour  and  a  half,  or  a  somewhat  more 
rapid  current  of  air  for  a  longer  time.  Cut  off  the  oxygen  or 
air  and  pass  carbon  dioxide  through  for  an  hour  or  more.  Cool 
the  copper  oxide  very  slowly  and  uniformly  in  the  current  of 
carbon  dioxide,  and  when  it  is  cold  close  the  tube  at  both  ends 
with  short  pieces  of  rubber  tubing  and  glass  plugs.  Both  the 
oxygen  (or  air)  and  the  carbon  dioxide  will  be  sufficiently  purified 


NITROGEN  277 

by  passing  them  through  a  saturated  solution  of  acid  sodium 
carbonate  and  then  over  calcium  chloride. 

Select  for  the  combustion  a  hard  glass  tube  with  an  internal 
diameter  of  15  or  16  mm.  and  a  length  about  equal  to  that  of 
the  furnace.  Smooth  the  rough  edges  of  one  end  and  draw  out 
the  other  to  a  small  tube  about  100  mm.  long.  When  finished, 
each  end  should  project  50  or  60  mm.  beyond  the  furnace.  The 
large  end  of  the  tube  is  to  be  closed  with  a  tightly 
fitting  perforated  rubber  stopper  in  which  is 
inserted  a  small  glass  tube  so  bent  that  it  can 
be  joined  conveniently  to  a  SchifPs  azotometer. 
The  smaller  end  of  the  combustion  tube  is  to  be 
connected  with  an  apparatus  for  the  generation 
of  carbon  dioxide. 

The  azotometer,  Fig.  52,  in  which  the  nitro- 
gen is  collected,  is  to  be  filled  with  mercury  to 
a  point  somewhat  above  the  place  where  the  gas 
enters,  and  beyond  that  point  with  a  concen- 
trated solution  (1.35  sp.  gr.)  of  caustic  potash. 
The  apparatus  is  not  to  be  connected  with  the 
combustion  tube  until  nearly  all  of  the  air  has 
been  expelled  from  the  latter. 

To  fill  the  tube,  introduce : 

1.  A  loose  plug  of  ignited  asbestus. 

2.  A  layer,  about  125  mm.  in  length,  of  small  pieces  of  native 
magnesite  previously  dried  at  150°. 

3.  A  short  roll  of  oxidized  copper  wire  gauze. 

4.  A  layer  of  coarse  copper  oxide,  50  mm.  in  length. 

5.  A  layer  of  fine  copper  oxide,  75  mm.  in  length. 

6.  A   small   platinum   boat  containing  not  more  than   200 
milligrams  of  pure  uric  acid. 

7.  Enough  fine  copper  oxide  to  cover  the  boat.     The  acid 
and  the  fine  oxide,  after  the  introduction  of  the  latter,  are  to  be 
well  mixed  with  a  stiff  wire,  the  end  of  which  has  been  formed 
into  a  spiral  resembling  that  of  a  small  corkscrew. 


278  QUANTITATIVE  EXERCISES 

8.  A  layer  of  fine  copper  oxide,  75  or  100  mm.  in  length. 
This  is  introduced  without  removing  the  mixing  wire,  and  in  it  the 
wire  is  cleansed  from  any  of  the  acid  which  may  adhere  to  it. 

9.  Enough  coarse  copper  oxide  to  fill  the  tube  to  within  250 
mm.  of  the  end. 

10.  A  roll  of  copper  gauze,  125  mm.  in  length,  and  thick 
enough  to  fill  the  tube.     This  must  be  freed  from  oxide  by 
heating  to  redness  in  the  flame  of  the  blast  lamp  and  dropping 
it  while  hot  into  a  test  tube  containing  half  a  cubic  centi- 
meter of  pure  methyl  alcohol.     The  tube  is  immediately  closed 
with  a  cork  to  protect  the  hot  copper  from  the  air.     Afterwards 
it  is  heated  to  130°  in  an  air  bath  and  then  placed  in  a  desic- 
cator until  needed.     The  metallic  copper  serves  to  reduce  any 
oxides  of  nitrogen  which  are  formed  during  the  combustion. 

11.  A  layer  of  coarse  copper  oxide,  50  or  75  mm.  in  length, 
to  burn  any  carbon  monoxide  which  may  possibly  be  formed  by 
reduction  of  the  dioxide,  if  the  metallic  copper  contains  iron. 

12.  A  short  roll  of  wire  gauze  to  keep  the  last  portion  of 
copper  oxide  in  place. 

Place  the  filled  tube  in  the  furnace  and  immerse  the  end  of 
the  outlet  tube  in  a  small  quantity  of  mercury.  Attach  to  the 
other  end  of  the  combustion  tube  an  apparatus  for  the  genera- 
tion of  carbon  dioxide.  The  best  arrangement  for  this  purpose 
is  a  short  combustion  tube  filled  with  small  pieces  of  magne- 
site.  To  prepare  this,  a  tube  about  350  mm.  in  length  is 
closed  at  one  end,  filled  with  the  carbonate,  arid  drawn  out  at 
the  other  end  to  permit  its  being  attached  by  means  of  rubber 
tubing.  It  is  well  to  cover  the  magnesite  tube  by  slipping  over 
it  a  cylinder  of  sheet  iron  made  by  rolling  and  hammering  the 
metal  upon  an  iron  rod  of  the  proper  diameter.  The  covered 
tube  need  not  be  placed  in  a  furnace,  but  may  be  rested  upon 
the  ring  of  an  iron  stand.  Any  other  apparatus  for  the  genera- 
tion of  carbon  dioxide  may  be  used,  provided  it  is  in  a  condi- 
tion to  yield  the  gas  free  from  air,  which  is  not  the  case  with 
the  ordinary  Kipp  apparatus.  It  is  to  be  remembered  in  this 


NITROGEN  279 

connection  that  the  gas  obtained  by  dissolving  marble  in  acids 
always  contains  air,  and  that  neither  boiling  the  marble  under 
water  nor  exhausting  with  the  pump  suffices  to  liberate  the 
imprisoned  gas. 

Having  joined  the  two  tubes,  heat  the  one  containing  the 
magnesite  (throughout  its  whole  length,  but  most  strongly  at 
the  rear  end)  until  a  quite  rapid  current  of  carbon  dioxide  is 
obtained.  When  it  is  believed  that  all  the  air  has  been  expelled 
from  the  apparatus,  connect  it  with  the  azotometer  and  allow 
the  gas  to  ascend  through  the  caustic  potash.  After  a  few 
minutes,  open  the  stopcock  and  force  out  the  small  quantity  of 
gas  which  collects  at  the  top.  Close  it  again  and  observe 
whether  the  gas  which  now  enters  the  measuring  tube  is  com- 
pletely absorbed.  If  it  is  not,  or  if  the  quantity  of  air  which  col- 
lects at  the  top  is  sufficient  to  affect  the  results  appreciably,  lower 
the  reservoir  and  let  the  caustic  potash  run  into  it  by  opening  the 
stopcock.  Continue  to  heat  the  magnesite  and  repeat  the  test 
for  air  from  time  to  time  until  a  satisfactory  result  is  obtained. 

Pinch  the  rubber  connecting  the  two  tubes  and  detach  the 
rear  one,  inserting  in  its  place  a  tightly  fitting  glass  plug.  Light 
the  burners  under  the  copper  oxide  last  introduced  and  under 
the  roll  of  wire  gauze,  and  when  these  have  been  brought  to  a 
full  red  heat,  light  the  burner  under  the  oxide  nearest  the  mag- 
nesite. Advance  slowly  and  from  both  ends  with  the  heating 
until  all  of  the  copper  oxide  except  that  in  the  vicinity  of  the 
substance  to  be  burned,  as  well  as  the  roll  of  gauze,  is  at  a  red 
heat,  and  then  heat  the  remaining  oxide,  gently  at  first  and 
afterwards  strongly,  until  the  combustion  is  complete.  Finally, 
heat  the  magnesite  in  the  rear  end  of  the  tube  until  all  of  the 
nitrogen  has  been  driven  into  the  azotometer  tube. 

To  measure  the  gas,  connect  the  azotometer  by  means  of  a 
narrow  glass  tube  with  a  second  one  filled  with  water,  and 
transfer  the  nitrogen  to  the  latter  in  order  that  its  volume  may 
be  read  over  water  instead  of  over  caustic  potash,  whose  vapor 
tension  will,  of  course,  be  unknown. 


280  QUANTITATIVE  EXERCISES 

If  the  surface  of  the  metallic  copper  in  the  front  portion  of 
the  tube  was  in  good  condition,  i.e.  free  from  oxide,  and  if  the 
combustion  was  properly  managed,  the  gas  will  be  free  from 
nitric  oxide.  Otherwise  it  may  be  present,  and  the  volume, 
therefore,  too  large.  The  presence  of  nitric  oxide  in  nitrogen 
may  be  detected,  and  its  quantity  approximately  determined  in 
the  following  manner  :  The  azoto  meter  containing  the  gas  over 
the  alkali  is  connected  with  a  second  one  containing  some 
air  over  water.  Having  ascertained  the  volumes  of  both  gases 
under  atmospheric  pressure,  a  few  cubic  centimeters  of  air  are 
introduced  into  the  nitrogen.  If  the  latter  contains  nitric  oxide, 
it  will  be  converted  into  the  dioxide  which  will  then  be  absorbed 
by  the  alkali.  The  azotometer  is  shaken  and  the  gas  again  meas- 
ured. If  contraction  takes  place,  i.e.  if  the  final  volume  is  less 
than  the  sum  of  the  volumes  of  the  nitrogen  and  of  the  air 
introduced,  it  is  due  to  the  reactions 


2  NO2  +  2  KOH  =  H2O  +  KNO3  +  KNO2  ; 

and  the  volume  of  the  nitric  oxide  is  equal  to  two-thirds  of  the 
contraction  which  follows  the  introduction  of  the  air,  while 
the  volume  of  the  nitrogen  in  it  is  one-half  as  large.  Finally,  the 
gas  is  measured  over  water  by  transferring  it  to  the  azotometer 
from  which  the  air  was  taken.  A  correction  must,  of  course, 
be  made  for  the  oxygen  which  has  disappeared  in  converting 
the  NO  into  NO2.  When  the  air  is  passed  from  the  azotometer 
containing  water  into  that  containing  the  alkali,  a  slight  con- 
traction takes  place  owing  to  diminished  aqueous  tension  ;  hence 
the  method  is  not  adapted  to  the  detection  and  determination  of 
minute  quantities  of  nitric  oxide. 

The  following  modification  of  the  Dumas  method  is  to  be 
preferred  when  an  efficient  Sprengel  pump  is  available  :  A  com- 
bustion tube  of  suitable  length  and  diameter  is  closed  at  one  end. 
A  few  small  pieces  of  magnesite  are  dropped  into  it  and  confined 
in  place  by  a  plug  of  asbestus.  In  all  other  respects  the  tube 


NITROGEN  281 

is  filled  in  the  usual  manner.  The  open  end  is  then  drawn  out 
to  a  small  tube  70  or  80  mm.  in  length.  After  laying  the  filled 
tube  in  the  furnace,  it  is  connected  with  the  pump  by  means  of 
thick  rubber  tubing  securely  tied  with  waxed  shoemaker's  thread. 
The  tube  is  pumped  out  and  meanwhile  the  rubber  connection 
is  covered  occasionally  with  a  thin  coat  of  shellac.  When  a 
sufficient  vacuum  has  been  obtained,  the  magnesite  is  heated 
until  the  pressure  within  the  tube  is  restored.  A  gas  measuring 
tube  filled  with  a  solution  of  caustic  potash  is  then  inverted  over 
the  outlet  of  the  pump,  and  the  combustion  conducted  as  usual. 
After  it  is  finished  the  temperature  is  lowered  very  slowly  and 
uniformly  until  the  glass  becomes  sufficiently  rigid,  when  the 
burner  under  the  magnesite  is  closed  and  the  tube  pumped  out. 

III.   BY  VARRENTRAPP  AND  WILL'S  METHOD 
(Ruffle's  Modification) 

There  are  required  : 

1.  A  combustion  tube  with  an  internal  diameter  of  18  mm. 
and  a  length  of  575  mm.     One  end  is  to  be  drawn  out  to  a 
small  tube  and  then  closed. 

2.  A  Peligot  tube  in  which  to  absorb  ammonia.     The  absorp- 
tion tube  and  the  combustion  tube  are  each  fitted  with  a  per- 
forated stopper  and  are  connected  by  a  small  glass  tube. 

3.  Standard  solutions  of  sulphuric  acid  and  ammonia.     Those 
used  in  the  Kjeldahl  process  will  suffice. 

4.  A  mixture   of    equal    parts    by  weight   of   dry    calcium 
hydroxide  and  of  finely  powdered  sodium  thiosulphate  which 
has  been  dried  at  100°. 

5.  A  mixture  of  equal  parts  by  weight  of  finely  powdered 
sugar  and  flowers  of  sulphur. 

6.  Granulated  soda-lime. 

To  fill  the  combustion  tube,  introduce : 

(a)  A  loose  plug  of  asbestus  which  has  been  ignited  over  the 
blast  lamp. 


282  QUANTITATIVE  EXERCISES 

(b)  A  layer  of  the  lime-sulphur  mixture  25  to  30  mm.  in 
length. 

(c)  The  substance  mingled  with  the  mixtures  4  and  5.    Weigh 
out  a  quantity  of  any  nitrogenous  commercial  fertilizer  which  will 
yield  from  35  to  60  milligrams  of  nitrogen,  and  mix  it  thoroughly 
with  from  5  to  10  grams  of  the  sugar-sulphur  mixture.     Pour 
upon  a  glazed  paper  or  into  a  porcelain  mortar  a  quantity  of  the 
lime-thiosulphate  mixture  which  will  fill  the  tube  for  a  length  of 
250  mm.    Add  to  this  the  mixture  containing  the  substance  to  be 
analyzed.    Mix  thoroughly  and  pour  the  whole  into  the  combus- 
tion tube.     Rinse  the  paper  or  mortar  with  a  little  of  the  lime- 
thiosulphate  mixture,  and  shake  down  the  contents  of  the  tube. 

(d)  Introduce  enough  soda-lime  to  fill  the  tube  to  within  50 
or  60  mm.  of  the  end,  and  finally  a  plug  of  previously  ignited 
asbestus. 

Hold  the  tube  in  a  horizontal  position  and  tap  with  it  on  the 
table  until  a  sufficient  channel  for  the  escape  of  gas  is  produced 
along  its  whole  length.  Place  the  tube  in  the  furnace  with  the 
end  projecting  far  enough  to  insure  the  safety  of  the  stopper. 
Connect  it  with  the  absorption  apparatus  and  measure  into  the 
latter  50  or  75  cc.  of  0.2  normal  sulphuric  acid. 

Bring  the  front  portion  of  the  soda-lime  to  a  full  red  heat;  then 
beginning  with  the  burner  nearest  the  heated  part  of  the  tube, 
light  at  intervals  the  others  in  their  order  .until  the  combustion  of 
the  substance  begins.  Burn  slowly,  but  finally  bring  the  whole 
tube  to  redness.  When  the  liquid  in  the  absorption  apparatus 
rises  in  the  limb  towards  the  furnace,  attach  the  aspirator,  break 
off  the  small  end  of  the  combustion  tube,  and  draw  air  through  the 
apparatus  for  a  few  minutes.  Determine  the  excess  of  the  acid 
with  tenth-normal  ammonia,  using  cochineal  as  the  indicator. 

This  modification  of  the  Varrentrapp-Will  method  is  appli- 
cable to  mtro-compounds  and  nitrates.  In  its  original  form  the 
substance  was  decomposed  by  soda-lime  only,  and  the  applica- 
tion of  the  method  was  therefore  limited  to  those  substances  in 
which  the  nitrogen  is  not  in  combination  with  oxygen. 


CHAPTER  XII 
PHOSPHORUS  AND  ARSENIC 


EXERCISE  XXIX 

THE  DETERMINATION   OF  PHOSPHORIC  ACID  IN 
PHOSPHATE  ROCK 

There  are  required : 

1.  A  nitric  acid  solution  of  ammonium  molybdate,  which  may 
be  prepared  from  molybdenum  trioxide  or  from  the  commercial 
ammonium  molybdate.  (a)  To  prepare  it  from  the  former,  dis- 
solve 25  grams  of  the  trioxide  in  104  cc.  of  ammonia  of  0.96 
specific  gravity,  and  pour  the  solution,  slowly  and  with  constant 
stirring,  into  312  cc.  of  nitric  acid  of  1.2  specific  gravity.  Or, 
(b)  dissolve  37.5  grams  of  pulverized  commercial  ammonium 
molybdate  in  250  cc.  of  hot  water,  and  pour  the  solution,  as 
directed  above,  into  250  cc.  of  nitric  acid  of  1.2  specific  gravity. 
The  solution  prepared  from  the  trioxide  contains  about  5  per  cent 
of  molybdic  anhydride,  while  that  prepared  from  the  ammonium 
salt  will  contain  a  quantity  of  the  anhydride  ranging  between 
5.5  (for  (NH4)2  MoO4)  and  6.1  percent  (for  (NH4)6  Mo7O24  4  H2O), 
according  to  the  composition  of  the  salt  employed.  Whether 
the  solution  is  prepared  in  the  one  way  or  the  other,  it  should 
be  allowed  to  stand  for  several  days  in  a  moderately  warm  place 
in  order  that  any  phosphoric  acid  which  it  contains  may  be 
precipitated.  The  clear  solution  is  afterwards  decanted,  or 
siphoned  off,  and  kept  in  a  dark  place.  Exposed  to  the  light, 
it  is  slowly  decomposed  with  deposition  of  yellow  crystals  having 
the  composition  MoO3  2  H2O.  Fifty  cubic  centimeters  of  the 
reagent  are  added  for  each  decigram  of  P2O5  to  be  precipitated. 

283 


284  QUANTITATIVE  EXERCISES 

Owing  to  the  high  cost  of  molybdenum  compounds,  it  is 
desirable  to  recover  the  molybdic  acid  which  has  been  employed 
in  precipitating  phosphoric  acid.  To  do  this,  the  nitrates  and 
the  precipitates  containing  it  are  evaporated  to  dryness  and 
heated  until  nearly  all  of  the  ammonium  nitrate  has  been  de- 
composed. The  residue  is  digested  with  ammonia,  filtered,  and 
the  filtrate  treated  with  magnesia  mixture  (see  2,  below).  After 
standing  for  12  hours,  the  ammonium  magnesium  phosphate  is 
filtered  off,  the  filtrate  acidified  with  nitric  acid,  and  the  anhy- 
dride collected  upon  a  filter  and  washed  with  the  least  possible 
quantity  of  cold  water. 

2.  "  Magnesia  Mixture."  Dissolve  27.5  grams  of  crystallized 
magnesium  chloride,  MgCl2  6  H2O,  in  water.  Add  175  cc.  of 
ammonia  of  0.96  specific  gravity  and  dilute  to  a  liter.  Allow 
the  solution  to  stand  for  some  days,  and  then  filter,  if  necessary. 
Of  this  solution  10  cc.  are  to  be  added  for  each  decigram  of  P2O5 
to  be  precipitated. 

Treat  from  4  to  5  grams  of  finely  powdered  phosphate  rock 
in  a  porcelain  dish  with  60  cc.  of  concentrated  hydrochloric 
acid.  Digest  on  a  water  bath  for  an  hour  and  then  evaporate 
to  dryness.  Moisten  the  residue  with  strong  hydrochloric  acid, 
again  evaporate  to  dryness,  and  finally  heat  for  an  hour  at  110°. 
This  treatment  will  render  the  silica  anhydrous  and  insoluble. 
Add  10  cc.  of  nitric  acid  of  1.2  specific  gravity  and  evaporate 
to  dryness.  Repeat  the  treatment  with  nitric  acid  and  the  sub- 
sequent evaporation  until  all  hydrochloric  acid  is  removed. 
Digest  the  residue  for  a  considerable  time  on  the  water  bath 
with  water  containing  10  cc.  of  dilute  nitric  acid.  Filter  into 
a  half-liter  measuring  flask,  wash  the  residue  and  the  paper 
with  water,  and  dilute  the  filtrate  to  the  mark. 

To  25-cc.  portions  of  the  phosphoric  acid  solution  add  75  cc. 
of  the  molybdate  reagent.  Warm  for  six  hours  at  a  tempera- 
ture of  50°.  Take  out  a  small  portion  of  the  clear  liquid,  add 
to  it  an  equal  volume  of  the  molybdate  solution,  and  warm  for 
some  time  at  50°.  If  a  precipitation  of  phosphoric  acid  follows, 


PHOSPHORUS  AND  ARSENIC  285 

add  more  molybdate  to  the  main  solution  and  continue  to  heat 
for  several  hours  at  50°.  When  the  precipitation  is  found  to 
be  complete,  filter  through  a  small  paper.  Wash  the  precipi- 
tate, without  attempting  to  bring  all  of  it  upon  the  paper, 
with  a  solution  consisting  of  10  parts  of  the  molybdate,  2  parts 
of  nitric  acid  (1.2  sp.  gr.),  and  8  parts  of  water,  until  the  filtrate 
gives  no  reaction  for  calcium  when  treated  with  a  little  sulphuric 
acid  and  twice  its  volume  of  alcohol.  Instead  of  the  solution 
given  above,  one  of  25  grams  of  ammonium  nitrate  in  250  cc. 
of  water  may  be  used  to  wash  the  precipitate.  Dissolve  the 
precipitate  in  the  beaker  and  on  the  filter  in  the  least  possible 
quantity  of  dilute  ammonia  by  passing  a  small  quantity  of  the 
liquid  through  the  paper  repeatedly  until  a  perfect  solution 
is  obtained.  Dilute  the  filtrate  with  a  little  water  and  pass 
it  again  through  the  paper  into  a  200-cc.  beaker.  Wash  the 
beaker  in  which  the  phosphoric  acid  was  precipitated,  and  the 
filter  with  dilute  ammonia  (1  part  of  ammonia  to  3  parts  of 
water).  Add  dilute  hydrochloric  acid  until  the  precipitate 
begins  to  dissolve  somewhat  slowly  on  stirring  the  liquid,  and 
then  add  from  6  to  8  cc.  of  ammonia  (0.96  sp.  gr.).  Let  the 
solution,  whose  volume  at  this  stage  should  not  exceed  70  or 
75  cc.,  cool  to  the  temperature  of  the  room.  Add  from  a  burette, 
slowly  and  with  constant  stirring,  from  15  to  20  cc.  of  the 
magnesia  mixture,  and  then  a  quantity  of  ammonia  (0.96 
sp.  gr.)  equal  to  about  one-third  of  the  volume  of  the  liquid 
in  the  beaker.  After  six  hours  collect  the  precipitate  on  a 
filter,  using  the  filtrate  to  bring  into  the  paper  the  portion 
which  clings  to  the  glass.  Wash  with  a  solution  of  1  part  of 
ammonia  (0.96  sp.  gr.)  in  3  parts  of  water  until  the  filtrate  gives 
no  reaction  for  chlorine.  Dry  the  filter  and  remove  the  pre- 
cipitate to  a  weighed  porcelain  crucible.  Burn  the  paper  in  a 
platinum  wire.  Heat  the  crucible  for  some  time  over  a  Bunsen 
burner  and  then  two  or  three  minutes  over  the  blast  lamp.  If 
the  material,  after  ignition,  is  not  perfectly  white  in  color,  add 
a  few  drops  of  nitric  acid,  evaporate  to  dryness,  heat  over  a 


286  QUANTITATIVE  EXERCISES 

Bunsen  burner,  and  then  again  over  the  blast  lamp.  Treat  the 
filtrate  with  more  magnesia  mixture  in  order  to  ascertain 
whether  the  precipitation  was  complete.  Dissolve  the  precipi- 
tate in  hydrochloric  acid.  If  the  solution  is  cloudy,  silica  is 
probably  present,  and  its  weight  must  be  determined  and  de- 
ducted from  the  apparent  weight  of  the  pyrophosphate.  For  this 
purpose  filter,  wash  thoroughly  with  water  containing  a  little 
hydrochloric  acid,  and  burn  the  paper  in  a  weighed  platinum 
crucible.  Finally,  in  order  to  ascertain  whether  any  molyb- 
denum was  precipitated  with  the  phosphoric  acid,  treat  the  acid 
solution  of  the  pyrophosphate  with  hydrogen  sulphide. 


The  composition  of  ammonium  phosphomolybdate  is  given 
by  Rammelsberg  as  (NH4)3PO4(MoO3)116  H2O,  but  according 
to  Hundeshagen  (Zeitschrift  fur  analytische  Chemie,  28,  14), 
the  compound,  however  precipitated,  has  the  composition 
(NH4)3PO4(MoO3)12,  when  dried  at  130°.  Precipitated  from  a 
nitric  or  hydrochloric  acid  solution  and  undried,  it  is  said  to 
hold  in  loose  combination  two  molecules  of  these  acids  and  one 
of  water.  The  substances  whose  presence  should  be  avoided  in 
solutions  from  which  phosphoric  acid  is  to  be  precipitated  by 
ammonium  molybdate  are :  (1)  arsenic  and  silica,  which  may 
also  be  precipitated ;  (2)  all  compounds  which,  like  hydriodic 
acid  and  many  organic  compounds,  reduce  molybdic  acid ; 
(3)  organic  acids,  such  as  tartaric  and  citric,  in  the  presence  of 
which  the  precipitation  is  incomplete ;  (4)  hydrochloric  acid  and 
chlorides  which  dissolve  the  precipitate  to  some  extent  even 
in  the  presence  of  an  excess  of  ammonium  molybdate.  The 
phosphomolybdate  is  also  somewhat  soluble  in  water  and  in 
dilute  nitric  and  sulphuric  acids,  but  its  solubility  in  these  is 
greatly  diminished  by  the  presence  of  an  excess  of  ammonium 
molybdate.  It  is  for  this  reason  that  the  latter  is  added  to  the 
water  with  which  the  precipitate  is  washed.  Its  solubility  is 
likewise  lessened  by  the  presence  of  ammonium  nitrate,  hence 


pHosriioius  AND  ARSENIC  287 

the  frequent  use,  for  economical  reasons,  of  a  ten  per  cent  solu- 
tion of  this  salt  as  a  wash  liquid. 

Ammonium  magnesium  phosphate,  NH4MgPO4  6H2O,  loses 
five  molecules  of  water  at  100°.  At  higher  temperatures  it  is 
converted  into  magnesium  pyrophosphate  with  loss  of  water 
and  ammonia.  It  is  soluble  in  15,300  parts  of  water  at  ordinary 
temperatures  (Fresenius),  but  nearly  insoluble  in  water  contain- 
ing 2.5  per  cent  of  ammonia  (NH3).  Its  solubility  is  in  all  cases 
increased  by  ammonium  chloride,  but  to  a  less  extent  in  the 
presence  of  an  excess  of  magnesia  mixture.  The  conversion  of 
the  salt  into  magnesium  pyrophosphate  should  be  effected  in 
a  porcelain,  and  not  in  a  platinum,  crucible. 

Metaphosphoric  and  pyrophosphoric  acids  and  their  salts  must 
be  converted  into  orthophosphoric  acid  or  orthophosphates  before 
attempting  to  precipitate  with  ammonium  molybdate  or  with 
magnesia  mixture.  This  transformation  may  be  effected  in  two 
ways:  (1)  by  long-continued  fusion  with  4  or  6  parts  of  potas- 
sium sodium  carbonate;  and  (2)  by  warming  for  a  long  time 
with  a  strong  mineral  acid.  The  first  method  is  applicable  only 
to  those  metaphosphates  and  pyrophosphates  which  are  com- 
pletely decomposed  by  fusion  with  alkaline  carbonates.  In 
other  cases  —  those,  for  example,  of  the  metaphosphates  and 
pyrophosphates  of  the  alkaline  earths  —  the  second  method  must 
be  employed. 

EXERCISE  XXX 

THE  DETERMINATION  OF  PHOSPHORUS  IN  IRON  BY  THE 
STOECKMANN-FRESENIUS  METHOD 

Treat  about  5  grams  of  pig-iron  drillings  in  a  liter  flask,  in 
the  neck  of  which  a  funnel  has  been  placed,  with  60  cc.  of 
nitric  acid  of  1.2  specific  gravity,  adding  the  acid  in  small  por- 
tions from  time  to  time  and  heating  the  flask.  When  the 
frothing  ceases,  boil  the  liquid  gently  until  the  iron  is  dissolved. 
Transfer  the  contents  of  the  flask,  together  with  the  rinsings, 


288  QUANTITATIVE  EXERCISES 

to  a  200-cc.  porcelain  dish  and  evaporate  to  a  small  volume. 
Add  5  grams  of  ammonium  nitrate  and  continue  the  evaporation 
to  dryness  on  a  sand  bath,  stirring  the  contents  of  the  dish  from 
time  to  time.  Heat  the  dried  residue  over  the  naked  flame  until 
the  carbon  is  burned  and  the  ammonium  nitrate  is  destroyed. 
Digest  with  hot  concentrated  hydrochloric  acid  until  the  iron 
oxide  is  dissolved.  Dilute,  filter  out  the  silica,  wash  the  paper, 
and  evaporate  the  filtrate  repeatedly  with  nitric  acid  until  all  the 
hydrochloric  acid  has  been  removed.  Precipitate  the  phosphoric 
acid,  first  with  ammonium  molybdate  and  then  with  magnesia 
mixture,  as  directed  in  Exercise  XXIX.  The  ammonium-magne- 
sium phosphate  may  contain  arsenic.  To  remove  this,  dissolve  in 
a  little  hydrochloric  acid,  heat  the  solution  to  70°,  and  treat  with 
hydrogen  sulphide.  Filter,  wash  the  paper,  concentrate  the  fil- 
trate, and  precipitate  the  phosphoric  acid  by  adding  the  requisite 
amount  of  ammonia  and  a  little  magnesia  mixture.  Proceed 
with  the  precipitate  as  directed  in  the  preceding  exercise. 

If  the  residue  of  silica  (see  above)  is  not  white,  fuse  it,  together 
with  the  filter  ash,  with  sodium  carbonate,  dissolve  with  dilute 
nitric  acid,  and  proceed  in  the  same  manner  as  with  the  nitric 
acid  solution  of  the  iron,  i.e.  evaporate  with  ammonium  nitrate, 
digest  the  residue  with  hydrochloric  acid,  filter,  evaporate  with 
nitric  acid,  precipitate  with  ammonium  molybdate,  etc. 


EXERCISE  XXXI 

THE  DETERMINATION  OF  PHOSPHORIC  ACID  IN 
FERTILIZERS 

(Method  of  the  Association  of  Official  Agricultural  Chemists) 

The  reagents  required  are  : 

1.  Ammonium  molybdate,  prepared  as  directed  in  Exercise 
XXIX. 

2.  Magnesia  mixture,  prepared  as  directed  in  Exercise  XXIX. 
It  may  also  be  made  by  dissolving  5.5  grams  of  recently  ignited 


PHOSPHORUS  AND  ARSENIC  289 

magnesium  oxide  in  dilute  hydrochloric  acid  (avoiding  an  excess 
of  the  latter),  boiling  the  solution  for  a  few  minutes  with  a  small 
additional  quantity  of  the  oxide  to  precipitate  iron,  aluminium, 
and  phosphoric  acid,  filtering,  adding  70  grams  of  ammonium 
chloride  and  175  cc.  of  ammonia  of  0.96  specific  gravity,  and 
diluting  with  water  to  half  a  liter. 

3.  Dilute  ammonia,  prepared  by  adding  1  part  of  ammonia  of 
0.96  specific  gravity  to  3  parts  of  water,  or  1  part  of  concen- 
trated ammonia  to  6  parts  of  water. 

4.  A  solution  of  ammonium  nitrate,  prepared  by  dissolving 
50  grams  of  the  salt  in  water  and  diluting  the  solution  to  half 
a  liter. 

5.  A  solution  of  ammonium  citrate,  made  by  dissolving  92.5 
grams  of  citric  acid  in  375  cc.  of  water,  nearly  neutralizing  with 
ammonia,  cooling,  adding  ammonia  until  the  solution  is  exactly 
neutral  (using  a  saturated  alcoholic  solution  of  corallin  as  the 
indicator),  and  finally  diluting  to  half  a  liter.     The  solution 
should  have  a  specific  gravity  of  1.09  at  20°. 

6.  A  solution  of  magnesium  nitrate,  made  by  dissolving  80 
grams  of  calcined  magnesia  in  nitric  acid,  avoiding  an  excess 
of  the  latter,  boiling  the  solution  with  a  small  excess  of  mag- 
nesium oxide,  filtering,  and  diluting  to  half  a  liter. 

a.    Determination  of  the  Water-Soluble  Phosphoric  Acid 

Place  2  grams  of  the  well-mixed  and  finely  pulverized  ferti- 
lizer in  a  filter  90  mm.  in  diameter,  and  wash  with  successive 
small  portions  of  water,  allowing  each  portion  to  pass  through 
the  paper  before  adding  another,  until  the  volume  of  the  fil- 
trate is  250  cc.  If  the  filtrate  is  turbid,  add  a  little  nitric  acid. 
Dilute  to  any  convenient  volume,  mixing  well,  and  determine 
the  phosphoric  acid,  as  directed  below,  in  measured  portions  of 
the  solution  which  correspond  to  0.5  gram  of  the  original  mate- 
rial. To  the  hot  liquid  add  50  cc.  of  ammonium  molybdate 
for  each  decigram  of  P2O5  supposed  to  be  present.  Digest  at 


290  QUANTITATIVE  EXERCISES 

65°  for  one  hour.  Filter  and  wash  the  precipitate  with  the 
ammonium  nitrate  solution  4.  Add  more  ammonium  molyb- 
date  to  the  nitrate  and  digest  again  at  65°  to  determine  whether 
the  precipitation  is  complete.  Dissolve  the  precipitate  on  the 
filter  with  ammonia  and  hot  water.  Wash  into  a  beaker  to  a 
volume  not  exceeding  100  cc.  Nearly  neutralize  with  hydro- 
chloric acid.  Cool  and  add  magnesia  mixture  from  a  burette 
at  the  rate  of  one  drop  per  second,  stirring  vigorously.  After 
15  minutes  add  30  cc.  of  ammonia  of  0.95  specific  gravity. 
Allow  the  beaker  to  stand  for  two  or  three  hours,  then  collect 
the  precipitate  on  a  filter  and  wash  it  with  the  dilute  ammonia  3. 
Transfer  the  precipitate  to  a  weighed  porcelain  crucible,  burn 
the  paper,  and  ignite,  first  over  the  Bunsen  burner  and  finally 
for  ten  minutes  over  a  blast  lamp. 

b.    Determination  of  the  Citrate-Insoluble  Phosphoric  Acid 

Measure  100  cc.  of  the  ammonium  citrate  solution  5  into  a 
strong  200-cc.  flask,  and  place  the  flask,  loosely  stoppered,  in  a 
warm  water  bath.  When  the  temperature  of  the  solution  has 
risen  to  65°,  drop  into  it  the  paper  containing  the  water-insoluble 
phosphate.  Close  the  flask  tightly  with  a  smooth  rubber  stopper 
and  shake  violently  until  the  filter  paper  is  reduced  to  a  pulp. 
Return  the  flask  to  the  bath  and  maintain  the  temperature  of 
the  citrate  solution  at  exactly  65°,  shaking  the  flask  every  five 
minutes.  At  the  expiration  of  one-half  hour  from  the  time 
of  introducing  the  paper,  remove  the  flask  and  filter  its  con- 
tents as  rapidly  as  possible.  Wash  the  residue  thoroughly  with 
water  having  a  temperature  of  65°.  Transfer  the  filter  with 
its  contents  to  a  crucible  and  ignite  until  all  organic  matter  is 
destroyed.  Digest  the  residue  with  from  10  to  15  cc.  of  strong 
hydrochloric  acid  until  the  phosphate  is  dissolved.  Dilute  to 
200  cc.  and  determine  the  phosphoric  acid  in  measured  portions 
of  the  solution,  as  directed  under  «,  i.e.  by  precipitating  first 
with  ammonium  molybdate  and  then  with  magnesia  mixture, 


PHOSPHORUS  AND  ARSENIC  291 

etc.  Instead  of  burning  off  the  organic  matter,  as  directed 
above,  it  may  be.  destroyed  by  one  of  the  methods  given  under  c. 
In  the  case  of  "  non-acidulated  goods,"  the  removal  of  the 
water-soluble  phosphoric  acid  may  be  omitted.  If,  as  in  bone, 
fish,  etc.,  considerable  animal  matter  is  present,  the  residue,  after 
treatment  with  ammonium  citrate,  is  to  be  freed  from  organic 
matter  by  one  of  the  processes  given  under  c. 

c.    Determination  of  Total  Phosphoric  Acid 

To  destroy  the  organic  matter,  treat  2  grams  of  the  material 
by  one  of  the  following  methods. 

1.  Evaporate  with  5  cc.  of  the  magnesium  nitrate  solution  6. 
Ignite,  and  dissolve  the  residue  in  hydrochloric  acid. 

2.  Boil  with  from  20  to  30  cc.  of  strong  sulphuric  acid,  adding 
from  2  to  4  grams  of  sodium  nitrate  at  the  beginning  of  the 
digestion,  and  another  small  portion  when  the  liquid  has  become 
nearly  colorless,  or  adding  the  nitrate  in  small  portions  from 
time  to  time.     A  Kjeldahl  flask  graduated  to  250  cc.  is  recom- 
mended for  the  digestion.    When  the  solution  has  become  color- 
less, add  150  cc.  of  water,  boil  for  a  few  minutes,  cool,  and 
dilute  to  the  mark  with  water. 

3.  Digest  with  strong  sulphuric  acid  and  such  other  reagents 
as  are  used  in  the  Kjeldahl  or  the  Gunning  method  for  the 
determination  of  nitrogen.    Do  not  add  potassium  permanganate, 
but  when  the  solution  has  become  colorless,  add  about  100  cc. 
of  water  and  boil  for  a  few  minutes.     Cool  and  dilute  to  any 
convenient  volume.     This  method  is  advantageous  when  both 
the  phosphoric  acid  and  the  nitrogen  are  to  be  determined  in  a 
fertilizer. 

4.  Dissolve  in  30  cc.  of  concentrated  nitric  acid,  adding  a 
little  hydrochloric  acid. 

5.  Add  30  cc.  of  strong  hydrochloric  acid,  heat,  and  treat 
cautiously  from  time  to  time  with  small  quantities  of  pulverized 
potassium  chlorate. 


292  QUANTITATIVE  EXERCISES 

6.  Dissolve  with  from  15  to  30  cc.  of  strong  hydrochloric 
acid  and  with  from  5  to  10  cc.  of  concentrated  nitric  acid.  This 
method  is  recommended  for  fertilizers  containing  considerable 
iron  or  aluminium  phosphate. 

Boil  until  all  phosphates  are  dissolved  and  all  organic  matter 
is  destroyed.  If  hydrochloric  or  sulphuric  acid  has  been  used 
as  a  solvent,  add  15  grams  of  ammonium  nitrate.  Cool  and 
dilute  to  200  or  250  cc.  Mix  well  and  filter  through  a  dry 
paper.  Measure  off  portions  of  the  filtrate  corresponding  to 
0.25  or  0.5  gram  of  the  original  material,  neutralize  with  ammo- 
nia, and  dissolve  with  nitric  acid  any  precipitate  which  appears. 
Determine  the  phosphoric  acid  in  the  usual  manner. 

The  citrate-soluble  phosphoric  acid  is  the  difference  between 
the  total  and  the  sum  of  the  water-soluble  and  the  citrate- 
insoluble  acids,  and  the  "  available  "  phosphoric  acid  is  the  sum 
of  the  water-soluble  and  the  citrate-soluble  acids. 


d.  "  Optional  Volumetric  Method  "  for  the  Determination  of  Phos- 
phoric Acid  in  Fertilizers 

The  required  reagents  are  : 

1.  A  molybdic  solution  prepared  as  previously  directed.     To 
each  100  cc.  of  it  add  5  cc.  of  nitric  acid  of  1.42  specific  grav- 
ity.    This  solution  should  be  filtered  each  time  before  using. 

2.  Potassium  nitrate  or  ammonium  nitrate  solution  for  washing. 
To  prepare  this,  dissolve  3  grams  of  the  salt  in  100  cc.  of  water. 

3.  Nitric  acid  for  washing.     Dilute  100  cc.  of  nitric  acid  of 
1.42  specific  gravity  to  a  liter. 

4.  A  standard  solution  of  potassium  hydroxide  free  from  car- 
bonate, which  contains  18.17106  grams  of  KOH  in  a  liter.    One 
cubic  centimeter  of  it  should  neutralize  0.3238  cc.  of  a  normal 
acid,  and  is  equivalent  to  1  milligram  of  P2O5. 

5.  A  standard  solution  of  nitric  acid  equivalent  to  that  of 
potassium    hydroxide,  or    one-half    as    concentrated.     Phenol- 
phthalein  is  to  be  employed  as  the  indicator. 


PHOSPHORUS  AND  ARSENIC  293 

6.  An  alcoholic  solution  of  phenolphthalein. 

A  part  of  the  solution  made  as  directed  under  c  may  be 
employed  for  the  determination. 

For  percentages  of  5  or  below,  use  a  volume  corresponding 
to  0.4  gram  of  the  substance;  for  percentages  between  5  and 
20,  use  a  volume  corresponding  to  0.2  gram  of  the  substance. 
If  the  percentage  of  phosphoric  acid  is  above  20,  use  a  volume 
corresponding  to  0.1  gram  of  the  material. 

Add  from  5  to  10  cc.  of  nitric  acid  or  an  equivalent  quantity 
of  ammonium  nitrate,  nearly  neutralize  with  ammonia,  dilute  to 
from  75  to  100  cc.,  and  heat  in  the  water  bath  to  60°  or  65°. 
Add  25  or  35  cc.  of  filtered  ammonium  molybdate,  according  as 
the  percentage  of  phosphoric  acid  is  below  or  above  5.  Stir, 
and  allow  to  stand  for  fifteen  minutes.  Filter  immediately  there- 
after, wash  once  or  twice  by  decantation  (using  from  25  to  30  cc. 
of  water  each  time,  agitating  the  precipitate  thoroughly,  and 
allowing  it  to  settle),  and  then  transfer  to  the  filter.  Wash  five  or 
six  times  with  water,  or  until  the  entire  filtrate  reaches  a  volume 
of  200  cc.  Transfer  the  filter  with  its  contents  to  a  beaker,  dis- 
solve in  a  small  excess  of  the  standard  potassium  hydroxide,  add 
phenolphthalein,  and  titrate  with  the  standard  nitric  acid. 

Other  Volumetric  Methods  for  the  Determination  of 
Phosphoric  Acid 

Free  phosphoric  acid  may  be  determined  by  means  of  a  stand- 
ard alkali,  with  use  of  methyl  orange  or  phenolphthalein,  or, 
better,  with  use  of  both  of  these  indicators. 

If  alkali  is  added  to  a  solution  containing  the  free  acid  and 
methyl  orange,  the  color  changes  from  red  to  yellow  with  the 
formation  of  the  di-hydrogen  salt, 

H3PO4  +  KOH  =  H2O  +  H2KPO4. 

In   other   words,  the   di-hydrogen   phosphates  are  neutral  to 
methyl  orange. 


294  QUANTITATIVE  EXERCISES 

If  phenolphthalein  is  subsequently  added  to  the  solution  and 
the  titration  with  alkali  is  continued,  the  red  color  of  this  indi- 
cator appears  only  after  the  conversion  of  the  di-hydrogen  phos- 
phate into  the  mono-hydrogen  salt, 

H2KP04  +  KOH  =  H20  +  HK2PO4. 

It  will  be  seen  that  these  reactions  may  be  employed  for  the 
determination  of  the  phosphoric  acid  in  the  phosphates  of  the 
alkalies ;  for,  if  to  a  solution  containing  such  a  salt  and  methyl 
orange  a  mineral  acid  is  added,  the  red  color  of  the  indicator 
will  appear  only  when  all  of  the  phosphate  has  been  converted 
into  the  di-hydrogen  salt,  and  this  may  then  be  titrated  with  a 
standard  alkali  and  phenolphthalein. 

With  certain  modifications,  the  method  is  likewise  applicable 
to  the  determination  of  phosphoric  acid  in  the  phosphates  of 
the  alkaline  earths,  magnesium,  iron,  aluminium,  etc.  The 
liquid  containing  the  phosphate  in  solution  and  an  excess  of  the 
acid  in  which  it  was  dissolved  is  treated  with  methyl  orange 
and  then  carefully  neutralized  with  potassium  or  sodium  hydrox- 
ide, leaving  the  phosphoric  acid  in  the  form  of  di-hydrogen 
phosphate.  There  are  then  added  phenolphthalein,  an  excess 
of  calcium  chloride,  and  finally  a  standard  alkali  until  the  red 
color  of  the  indicator  appears.  The  reactions  which  take  place 
may  be  represented  by  the  equation 

CaH4(PO4)2  +  2  CaCl2  -!-  4  NaOH  =  Ca32  PO4  +  4  NaCl  +  4  H2O. 

The  precipitate  may  be  redissolved  and  the  determination 
repeated.  The  addition  of  an  excess  of  calcium  chloride  is 
essential,  since  otherwise  the  reactions  are  in  part  as  follows : 

2  CaH4(P04)2  +  2  CaCl2  +  6  KOH  =  Ca3  2  PO4  +  CaHPO4  + 
K2HPO4  +  6  H2O  +  4  KC1. 

The  reaction  which  takes  place  when  the  phosphoric  acid  is  in  com- 
bination with  iron  or  aluminium  may  be  represented  as  follows : 

Fe(H2PO4)3  +  3  CaCl2  +  6  NaOH  =  FePO4  +  Ca3  2  PO4  + 
6  NaCl  +  6  H2O. 


PHOSPHORUS  AND  ARSENIC  295 

The  same  relative  quantity  of  alkali  is  required  for  the  precipi- 
tation of  the  phosphoric  acid  as  in  the  case  of  phosphates  free 
from  iron  and  aluminium.  If  the  phosphoric  acid  is  wholly  in 
the  form  of  the  iron  or  aluminium  salt,  the  addition  of  calcium 
chloride,  before  titrating  with  a  standard,  alkali  and  phenol- 
phthalein,  is  unnecessary;  for 

3  FePO4  +  6  HC1  =  Fe(H2PO4)3  +  2  FeCl3,  and 

Fe(H2PO4)3  +  2  FeCl3  +  6  KOH  =  3  FePO3  +  6  KC1  +  6  H2O. 

Another  method  still  in  use  for  the  volumetric  determination 
of  phosphoric  acid  is  based  on  the  fact  that  the  uranyl  salts 
precipitate  the  acid  in  the  form  of  a  phosphate  of  definite  com- 
position, which  is  not  decomposed  by  potassium  ferrocyanide, 
and  which  does  not  therefore  give  with  that  reagent  the  charac- 
teristic color  reaction  of  uranium  salts. 

If  uranyl  acetate  is  added  to  a  solution  of  a  phosphate  con- 
taining no  other  free  acid  than  acetic  acid,  the  reaction  which 
follows  may  be  represented  by  the  equation 

Na2HP04  +  Ur02(C2H302)2  =  UrO2HPO4  +  2  NaC2H3O2. 

If  ammonia  or  ammonium  salts  are  present,  the  precipitate  has 
the  composition  UrO2NH4PO4,  in  which  the  ratio  of  Ur2  to 
PO4  is  the  same  as  in  the  salt  UrO2HPO4.  Both  salts  are 
insoluble  in  acetic  acid,  and  neither  gives  a  color  reaction  with 
potassium  ferrocyanide. 

In  practice,  the  solution  of  the  phosphate  is  treated  with 
sodium  acetate  to  insure  the  neutralization  of  any  free  mineral 
acid  which  may  be  present,  and  then  with  the  standard  solution 
of  uranyl  acetate  until  a  drop  of  the  liquid,  removed  with  a 
glass  rod  and  brought  into  contact  with  a  drop  of  potassium 
ferrocyanide  solution  upon  a  porcelain  plate,  gives  the  color 
reaction.  It  is  better,  however,  to  proceed  with  the  titration 
until  a  strong  color  reaction  is  obtained,  and  then  to  titrate  back 
with  a  standard  solution  of  some  phosphate.  The  solution 
of  ferrocyanide  must  be  freshly  prepared;  and  since  the  color 


296  QUANTITATIVE  EXERCISES 

reaction  is  retarded  by  certain  substances,  the  conditions  under 
which  the  solutions  are  standardized  should  be,  as  nearly  as 
possible,  like  those  which  it  is  known  will  prevail  in  the  sub- 
sequent work.  The  uranyl  salt  (acetate  or  nitrate)  may  be 
standardized  against  microcosmic  salt,  if  only  alkaline  bases 
are  in  combination  with  the  phosphoric  acid  to  be  determined. 
If,  however,  as  usually  happens,  the  phosphoric  acid  is  com- 
bined with  calcium,  the  uranyl  solution  must  be  standardized 
against  calcium  phosphate.  This  precaution  is  rendered  neces- 
sary by  the  fact  that  uranyl  phosphate  is  inclined  to  carry 
down  with  it  a  small  quantity  of  calcium  phosphate.  The 
difficulty  may  also  be  overcome  by  titrating  the  solution  of 
calcium  phosphate  into  a  measured  quantity  of  the  standard 
uranyl  solution  until  the  liquid  no  longer  gives  the  color  reac- 
tion with  potassium  ferrocyanide.  The  uranium  method  is 
excluded  in  the  case  of  phosphates  containing  iron  or  alu- 
minium, owing  to  the  insolubility  of  the  phosphates  of  these 
metals  in  acetic  acid. 

Other  methods  of  more  limited  application  than  either  of  the 
foregoing  are  based  on  the  precipitation  of  phosphoric  acid 
(1)  by  lead  salts,  and  (2)  by  silver  salts. 

(1)  The  solution,  which  must  be  free  from  chlorides  and  sul- 
phates, is  treated  with  sodium  acetate  to  neutralize  any  nitric 
acid  which  may  be  present,  and  then  with  the  standard  solution 
of  lead  acetate  until  no  further  precipitation  occurs.     Or,  better, 
the  solution  of  phosphate  to  which  sodium  acetate  has  been 
added  is  treated  with  more  than  a  sufficient  quantity  of  the  lead 
salt,  and  the  excess  is  titrated  with  a  standard  solution  of  potas- 
sium chromate  until  the  liquid  gives  a  red  color  with  neutral 
silver  nitrate. 

(2)  The  solution,  which  should  be  as  nearly  neutral  as  possi- 
ble, is  treated  with  an  excess  of  a  standard  silver  nitrate.    After 
careful  neutralization  of  the  liquid  with  ammonia,  and  nitration, 
the  excess  of  the  silver  is  determined  by  the  method  of  Mohr 
or  by  that  of  Volhard. 


PHOSPHORUS  AND  ARSENIC  297 

Two  indirect  volumetric  methods  for  the  determination  of 
phosphoric  acid  have  been  proposed.  One  of  them  is  based  on 
the  reduction  of  uranyl  phosphate  or  uranyl-ammonium  phos- 
phate with  zinc  and  an  acid,  and  its  reoxidation  with  a  standard 
solution  of  potassium  permanganate,  while  the  other  depends  on 
a  similar  reduction  and  reoxidation  of  the  molybdophosphate  of 
ammonium. 


The  Determination  of  Phosphorus  in  Organic  Compounds 

The  preliminary  treatment  by  which  the  phosphorus  to  be 
determined  is  converted  into  phosphoric  acid  and  the  organic 
matter  destroyed  by  oxidation  is  similar  to  or  identical  with 
that  employed  in  the  determination  of  sulphur  in  organic  com- 
pounds. 

If  the  compound  is  not  volatile,  it  may  be  decomposed  by  the 
method  of  Liebig,  or  it  may  be  heated  in  a  glass  tube  with  a 
mixture  of  1  part  of  potassium  chlorate  and  8  parts  of  dry 
sodium  carbonate  (Kolbe's  method.  See  Fresenius,  Quantitative 
Analyse,  2,  74).  Volatile  compounds  may  be  treated  advan- 
tageously by  the  method  of  Carius  or  by  that  of  Sauer. 

If  the  solution  of  phosphoric  acid  obtained  by  any  of  the 
foregoing  methods  contains  no  metals  except  those  of  the  alka- 
lies, the  acid  may  be  precipitated  directly  by  magnesia  mixture 
and  determined  as  magnesium  pyrophosphate.  If  other  metals 
are  present,  the  acid  must  be  precipitated  by  ammonium  molyb- 
date,  dissolved  in  ammonia,  and  then  reprecipitated  by  magnesia 
mixture. 

When  organic  substances  containing  phosphorus  are  inciner- 
ated in  the  usual  manner,  a  portion  of  the  phosphorus  is  lost  by 
volatilization  notwithstanding  the  presence  of  ash-forming  (non- 
volatile) bases.  The  same  is  true  of  sulphur,  chlorine,  and  the 
alkalies.  Hence  when  animal  or  vegetable  matter  is  reduced  to 
an  ash  with  a  view  to  determining  its  inorganic  constituents, 
special  methods  of  incineration  must  be  employed. 


298  QUANTITATIVE   EXERCISES 

The  Separation  of  Phosphoric  Acid  from  Other  Substances 

The  fact  that  most  phosphates  are  insoluble  in  neutral  and 
alkaline  solutions,  while  nearly  all  of  them  are  dissolved  by 
acids,  renders  the  separation  of  phosphoric  acid  from  bases  and 
from  other  acids  somewhat  difficult.  In  many  cases  it  is  better 
first  to  precipitate  the  substances  from  which  it  is  to  be  sepa- 
rated, and  then  to  determine  the  acid  in  the  filtrate.  The  fol- 
lowing list  includes  the  reagents  most  frequently  employed  in 
the  separation  of  phosphoric  acid. 

1.  Ammonium  Molybdate  separates  the  acid  from  all  bases 
except  molybdenum  and  ammonia,  but  not  from  silica  and  arse- 
nic acids.     If  silica  is  present,  the  material  should  be  repeatedly 
evaporated  to  dryness  with  strong  hydrochloric  acid,  the  residue 
heated  for  some  time  at  110°,  and  then  evaporated  with  nitric 
acid  until  all  the  hydrochloric  acid  has  been   removed.     The 
filtered  solution  will  be  free  from  silica.     Owing  to  the  danger 
of  converting  the  acid  phosphates  into  the  pyro-  or  meta-  salts, 
the  residue  should  not  be  heated  above  110°.     Arsenic  should  be 
removed  by  treating  the  acid  solution  with  hydrogen  sulphide. 
If  there  are  present  silver,  lead,  mercury,  copper,  cadmium,  bis- 
muth, or  any  other  metals  which  are  precipitated  from  acid  solu- 
tions by  hydrogen  sulphide,  it  is  well  to  remove  them  before 
precipitating  the  phosphoric  acid  with  ammonium  molybdate. 

The  precipitate  of  ammonium  phosphomolybdate  is,  in  all 
cases,  to  be  dissolved  in  ammonia,  the  ammoniacal  solution 
treated  with  magnesia  mixture,  and  the  phosphoric  acid  finally 
determined  as  magnesium  pyrophosphate. 

2.  Mercurous  Nitrate  separates  phosphoric  acid  from  all  bases 
except  mercury.     It  .also  precipitates  meta-  and  pyrophosphoric 
acids.     The   phosphate  is   dissolved  in  a  moderate   excess   of 
nitric  acid  and  the  solution  treated  in  a  porcelain  dish  with  a 
quantity  of  mercury  which  is  somewhat  more  than  equivalent 
to  the  free  nitric  acid.     The  liquid  is  then  evaporated  to  dry- 
ness  on  the  water  bath.     The  residue  is  repeatedly  moistened 


PHOSPHORUS  AND  ARSENIC  299 

with  water  and  evaporated  until  the  vapors  arising  from  the 
dish  no  longer  contain  nitric  acid.  Hot  water  is  then  added. 
The  precipitate,  which  consists  of  mercurous  phosphate,  mer- 
cury, and  mercurous  oxynitrates,  is  collected  upon  a  small  filter, 
thoroughly  washed  and  dried.  The  contents  of  the  paper  are 
transferred  to  a  platinum  crucible  and  mixed  with  dry  sodium 
carbonate.  The  paper  is  rolled  into  a  compact  ball  and  buried 
under  the  carbonate  in  the  crucible.  The  crucible  is  heated 
under  a  hood  with  a  good  draught,  gently  for  half  an  hour  and 
then  to  full  redness.  The  contents  of  the  crucible  are  dissolved 
in  water,  the  solution  filtered,  if  necessary,  the  filtrate  slightly 
acidified  with  hydrochloric  acid,  and  the  phosphoric  acid  deter- 
mined as  magnesium  pyrophosphate. 

If  the  substance  contains  iron  and  aluminium,  these  are  pre- 
cipitated, in  part  at  least,  with  the  mercurous  phosphate.  The 
iron  phosphate,  however,  —  but  not  that  of  aluminium,  —  is 
decomposed  by  the  subsequent  fusion  with  sodium  carbonate. 
If  aluminium  is  present,  the  process  is  modified  in  the  follow- 
ing manner:  The  substance  is  dissolved  in  the  least  possible 
quantity  of  nitric  acid.  The  hot  solution  is  then  treated  (1) 
with  mercurous  nitrate,  (2)  with  a  little  mercuric  nitrate,  and 
(3)  with  caustic  soda  until  a  slight  but  permanent  red  precipi- 
tate appears.  The  precipitate,  which  now  contains  no  alumin- 
ium, is  to  be  treated  in  the  manner  previously  described. 

This  method  of  separating  phosphoric  acid  was  devised  by 
Rose.  It  is  employed  instead  of  the  molybdate  method  when 
the  presence  of  molybdenum  would  embarrass  the  subsequent 
separation  of  the  bases. 

3.  Tin  Oxide  separates  phosphoric  acid  from  the  metals  of 
the  alkaline  earths,  also  from  nickel,  cobalt,  manganese,  zinc, 
aluminium,   and  from  iron  when  present  in   small  quantities 
(Reissig).    For  the  details  of  the  method,  see  Fresenius,  Quanti- 
tative Analyse,  1,  406. 

4.  Silver  Nitrate  separates  phosphoric  acid  from  the  fixed 
alkalies  and  from  the  alkaline  earths.    The  material  is  dissolved 


300  QUANTITATIVE  EXERCISES 

in  water  or  in  the  least  possible  amount  of  nitric  acid  and  the 
solution  treated  first  with  silver  nitrate  and  then  with  silver 
carbonate  to  neutralize  the  excess  of  acid.  The  precipitate  is 
collected  upon  a  paper,  washed,  and  dissolved  in  nitric  acid. 
The  silver  is  precipitated  by  hydrochloric  acid,  and  the  phos- 
phoric acid  by  magnesia  mixture. 

5.  Lead  Acetate  also  separates  phosphoric  acid  from  the  fixed 
alkalies  and  from  the  alkaline  earths.     The  substance  is  dis- 
solved in  the  least  possible  quantity  of  nitric  acid  and  the  solu- 
tion treated  (1)  with  a  little  ammonium  chloride,  (2)  with  lead 
acetate,  and  (3)  with  lead  carbonate  to  neutralize  the  excess  of 
acid.     After  long  digestion  the  precipitate   is  collected  on  a 
paper,  washed,  and  dissolved  in  a  slight  excess  of  nitric  acid. 
The  lead  is  then  precipitated  by  sulphuric  acid  and  alcohol,  and 
the  phosphoric  acid  by  magnesia  mixture. 

6.  In  cases  where  it  is  desired  to  separate  phosphoric  acid 
from  the  alkalies  and  alkaline  earths,  rather  than  to  determine 
it,  the  acid  may  be  advantageously  precipitated  from  the  nearly 
neutral  solution  by  ferric  chloride  and  ammonium  acetate. 

7.  Hydrogen  Sulphide   separates    phosphoric    acid   from   all 
those  metals  which  are  quantitatively  precipitated  by  it  from 
acid  solutions.     It  is,  however,  generally  preferred  to  precipi- 
tate lead  by  sulphuric  acid  and  alcohol,  and  silver  by  hydro- 
chloric acid. 

8.  Ammonium   Sulphide    separates    cobalt,    manganese,    and 
zinc  from  phosphoric  acid.     The  hydrochloric  acid  solution  of 
these  metals  is  placed  in  a  flask  and  treated  with  tartaric  acid, 
ammonium    chloride,    ammonia,   and    ammonium    sulphide    in 
excess.     The  flask  is  then  closed  and  allowed  to  stand  in  a 
warm  place  until  the  liquid  above  the  precipitate  is  clear  and 
has  a  pure  yellow  color.     The  precipitate  is  collected  on  a  filter 
and  washed  with  water  containing  ammonium  sulphide.     The 
phosphoric  acid  in  the  filtrate  may  be  precipitated  with  magne- 
sia mixture  without  removing  the  excess  of  ammonium  sulphide. 
Owing  to  the  solubility  of  its  sulphide  in  ammonium  sulphide, 


PHOSPHORUS  AND  ARSENIC  301 

this  method  is  not  adapted  to  the  separation  of  nickel  from 
phosphoric  acid. 

9.  Sulphuric  Acid  separates  barium,  strontium,  calcium,  and 
lead  from  phosphoric  acid.  The  phosphates  are  dissolved  in 
hydrochloric  acid,  or,  if  lead  is  present,  in  nitric  acid,  and  the 
metals  precipitated  with  a  small  excess  of  sulphuric  acid.  If 
strontium,  calcium,  or  lead  are  to  be  separated,  alcohol  must  be 
added  to  render  the  precipitation  complete. 


EXERCISE  XXXII 

THE   DETERMINATION   OF   ARSENIC   AS   MAGNESIUM 
PYROARSENIATE 

Weigh  about  0.3  gram  of  pure  arsenious  oxide  into  a  400-  or 
500-cc.  flask.  Add  a  little  hydrochloric  acid  and  a  small  quan- 
tity of  potassium  chlorate.  Warm,  and  introduce  from  time  to 
time  other  small  portions  of  the  chlorate  until  the  liquid  has  a 
strong  odor  of  chlorous  acid.  Afterwards  continue  to  heat  gently 
until  the  odor  has  nearly  disappeared.  Transfer  the  liquid  to  a 
beaker,  make  it  strongly  alkaline  with  ammonia,  and  add  a  mod- 
erate excess  of  magnesia  mixture.  Stir  thoroughly  and  keep  in 
a  cold  place  for  twenty-four  hours  or  longer.  Collect  the  precipi- 
tate on  a  paper  filter,  using  small  portions  of  the  filtrate  for  the 
purpose,  and  wash  with  a  mixture  of  three  parts  of  water  and 
one  of  ammonia  until  the  filtrate  no  longer  gives  a  reaction  for 
chlorine.  Dry  at  a  temperature  under  100°  and  remove  the 
precipitate,  without  injuring  the  paper,  to  a  small  porcelain  dish 
or  a  piece  of  glazed  paper.  Return  the  filter  to  the  funnel. 
Moisten  well  with  dilute  nitric  acid,  and  then,  after  fifteen  or 
twenty  minutes,  wash  out  the  paper  with  small  portions  of  boil- 
ing water.  Evaporate  the  filtrate  in  a  weighed  porcelain  crucible 
on  the  water  bath.  Add  the  main  portion  of  the  precipitate  and 
heat  the  crucible  and  its  contents  in  the  air  bath  to  a  tempera- 
ture of  125°  or  130°.  Afterwards  heat  for  two  hours  on  a  sand 


302  QUANTITATIVE  EXERCISES 

bath,  then  for  an  equal  length  of  time  to  a  higher  temperature 
on  an  iron  plate,  and  finally  over  a  lamp,  with  gradually  rising 
temperature,  to  constant  weight.  The  expulsion  of  the  ammonia 
from  the  ammonium  magnesium  arseniate  must  be  effected  at 
comparatively  low  temperatures,  otherwise  some  of  the  acid 
may  be  reduced  with  loss  of  arsenic. 

The  trioxide  of  arsenic,  and  arsenious  compounds  generally, 
cannot  be  heated  with  hydrochloric  acid  without  loss  except  in 
the  presence  of  a  strong  oxidizing  agent. 

Like  phosphoric  acid,  arsenic  acid  may  be  precipitated  by 
ammonium  molybdate.  The  precipitate  —  ammonium  molybdo- 
arseniate  —  is  soluble  in  ammonia,  and  from  the  ammoniacal 
solution  the  arsenic  acid  may  be  precipitated  by  magnesia  mix- 
ture. It  may  also,  like  phosphoric  acid,  be  precipitated  quanti- 
tatively by  uranyl  acetate  or  nitrate. 

When  arsenic  in  solution  is  wholly  in  the  arsenious  condition, 
it  may,  with  certain  precautions,  be  precipitated  as  the  trisul- 
phide,  collected  upon  a  weighed  filter,  and  dried  to  constant 
weight  at  100°.  Owing  to  the  danger  of  contamination  with 
free  sulphur,  it  is,  however,  generally  better  to  convert  the 
sulphide  into  arsenic  acid  and  to  precipitate  this  finally  with 
magnesia  mixture. 


CHAPTER   XIII 
SILICATES 

1  Whatever  the  method  employed  to  decompose  a  silicate,  it 
must  first  be  converted  into  a  very  fine  powder,  and  the  more 
refractory  the  substance  is,  the  finer  must  be  the  powder.  In 
all  cases  a  high  degree  of  subdivision  greatly  facilitates  the 
decomposition.  The  material  to  be  analyzed  is  usually  reduced 
to  a  coarse  powder  by  crushing  it  with  a  hammer  upon  a  steel 
anvil  or  in  a  steel  mortar  and  is  then  ground,  a  small  portion  at 
a  time,  in  an  agate  mortar.  The  different  small  portions  are 
well  mixed  in  a  watch  glass  or  other  suitable  receptacle,  and 
the  material  is  then  reground  in  the  same  manner.  In  order  to 
remove  any  particles  of  steel  which  may  have  been  detached,  a 
magnet  is  passed  back  and  forth  through  the  powder  before 
grinding  it  in  the  agate  mortar.  Porcelain  and  glass  mortars 
should  not  be  used  when  the  substances  to  be  pulverized  are 
harder  than  ordinary  salts.  Substances  having  a  hardness  of  7 
or  more,  i.e.  equal  to  or  higher  than  that  of  quartz,  should  not 
be  ground  even  in  agate.  It  is  safer  to  pulverize  them  in  steel 
mortars  and  to  separate  the  finer  from  the  coarser  material  by 
sifting.  This  operation,  also  that  of  elutriation,  which  is  some- 
times resorted  to  for  the  reduction  of  refractory  substances,  is 
described  below. 

1.  Sifting.  A  piece  of  washed  and  well-dried  linen  is  placed 
over  the  mouth  of  a  stand  cylinder  and  somewhat  depressed  in 
the  center  so  as  to  form  a  baglike  receptacle  for  the  material 
to  be  sifted.  The  partially  pulverized  mineral  is  placed  in  the 
depression  and  the  cylinder  covered  with  a  piece  of  soft  leather. 
The  latter  is  tightly  stretched  over  the  top  and  secured  under 
the  rim  of  the  cylinder  by  tying  it  with  a  string  or  by  grasping 

303 


304  QUANTITATIVE  EXERCISES 

it  between  the  thumb  and  forefinger.  The  finer  material  is  then 
forced  through  the  sieve  by  tapping  upon  the  leather  cover. 
The  coarser  material  which  remains  upon  the  cloth  is  reground 
and  again  sifted  in  the  same  manner.  It  is  obvious  that  if  the 
substance  is  not  perfectly  homogeneous,  the  grinding  and  sift- 
ing must  be  continued  until  the  whole  of  the  material  has  been 
passed  through  the  cloth.  The  powder  which  collects  in  the 
bottom  of  the  cylinder  is  well  mixed  before  withdrawing  a 
portion  for  analysis. 

2.  Elutriation.  In  this  operation  advantage  is  taken  of  the 
fact  that  when  powdered  substances  are  suspended  in  a  liquid, 
the  larger  particles  sink  more  quickly  than  the  smaller  ones. 
The  material  is  ground  with  water  in  an  agate  mortar  and  trans- 
ferred with  the  aid  of  a  wash  bottle  to  a  beaker.  Here  it  is 
stirred  up  with  more  water  and  then  allowed  to  stand  about 
one  minute.  The  water,  together  with  the  finer  material  which 
remains  suspended  in  it,  is  poured  into  another  beaker,  while 
the  coarser  material  remaining  in  the  bottom  is  reground  and 
again  elutriated.  The  regrinding  of  the  sediment  must,  of 
course,  be  continued  until  the  whole  of  the  material  has  been 
reduced  to  a  uniformly  fine  condition.  When  the  water  con- 
taining the  elutriated  substance  has  become  perfectly  clear,  it  is 
decanted  and  the  residue  is  dried  in  the  beaker  and  well  mixed. 

Since  water  dissolves  to  some  extent,  or  attacks  nearly  all 
substances,  especially  when  they  are  in  a  state  of  fine  subdi- 
vision, the  process  of  elutriation  should  not  be  resorted  to 
unnecessarily. 

THE  DETERMINATION  OF  WATER  IN  SILICATES 

If  a  silicate  contains  nothing  volatile  except  water  and  does 
not  absorb  oxygen  when  heated  in  the  air,  the  determination  of 
water  in  it  is  very  simple.  It  is  then  necessary  only  to  ignite  a 
weighed  quantity  of  the  material  over  the  blast  lamp  and  ascer- 
tain the  loss  in  weight.  On  the  other  hand,  the  determination 


SILICATES  305 

is  more  difficult  if  the  mineral  contains  iron  in  the  ferrous  con- 
dition or  other  oxidizable  matter,  or  substances  which,  like 
chlorine,  fluorine,  boric  acid,  and  the  alkalies,  are  liable  to  form 
volatile  compounds  at  or  below  the  temperature  required  for 
the  expulsion  of  the  water.  In  such  cases  the  mineral  must  be 
mixed  with  some  substance  which  will  decompose  it  and  thus 
facilitate  the  liberation  of  the  water  and  at  the  same  time  retain 
the  otherwise  volatile  constituents.  The  water  must  then  be 
collected  in  a  weighed  absorption  apparatus.  A  few  of  the 
methods  which  are  employed  when  the  water  in  silicates  cannot 
be  determined  by  simple  ignition  are  given  below. 

1.  Sipcoecz's  Method.*  About  4  grams  of  finely  powdered 
potassium-sodium  carbonate  are  strongly  heated  in  a  platinum 
crucible  and  then  allowed  to  cool  to  60°  or  50°.  About  1  gram 
of  the  pulverized  and  dried  silicate  is  thoroughly  mixed  with 
the  carbonate  with  the  aid  of  a  platinum  wire.  The  mixture  is 
transferred  to  a  roomy  platinum  boat  and  the  crucible  rinsed  with 
a  little  of  the  dried  carbonate.  The  boat  is  then  placed  in  the 
middle  of  a  porcelain  tube  which  is  glazed  on  the  inside  and  has 
a  length  of  40  centimeters  and  an  internal  diameter  of  17  mm. 
The  porcelain  tube  is  placed  in  a  hot-air  bath  25  centimeters  in 
length  in  such  a  way  that  the  two  ends  project  equally.  The  ends 
are  closed  with  perforated  stoppers  through  which  are  passed 
short  pieces  of  glass  tubing.  To  one  of  these  is  attached  a  tube 
filled  with  calcium  chloride,  and  to  the  other  a  gas-purifying 
apparatus  consisting  of  a  cylinder  containing  strong  sulphuric 
acid  and  a  tube  filled  with  soda-lime.  The  latter  is  connected  in 
turn  with  a  gasometer  filled  with  air.  The  temperature  of  the 
bath  is  raised  to  120°  or  130°  and  a  moderately  rapid  current  of 
air  conducted  through  the  apparatus  for  an  hour.  The  porcelain 
tube  is  then  placed  in  a  combustion  furnace  and  the  calcium 
chloride  tube  replaced  by  a  weighed  U-tube  containing  glass 
beads  which  have  been  moistened  with  concentrated  sulphuric 
acid.  Air  is  again  conducted  through  the  apparatus  and  the 

*  Zeitsch.  anal.  Chem.,  17,  206. 


306  QUANTITATIVE  EXERCISES 

temperature  of  the  tube  in  the  vicinity  of  the  boat  is  raised  very 
gradually  to  a  full  red  heat.  After  maintaining  this  tempera- 
ture for  about  an  hour,  the  end  of  the  tube  nearest  the  absorp- 
tion apparatus  is  cautiously  heated  in  order  to  volatilize  any 
water  which  may  have  condensed  at  that  point.  The  U-tube  is 
then  detached  and  weighed. 

If  it  is  desired  to  use  the  same  material  for  the  determination 
of  other  constituents  than  water,  the  boat  should  be  loosely  cov- 
ered with  a  lid  which  may  be  readily  made  for  it  from  a  piece 
of  platinum  foil. 

2.  JannascTis  Method.*  The  finely  pulverized  material  is 
dried  over  calcium  chloride,  mixed  with  lead  chromate,  and 
heated  in  a  combustion  tube  through  which  a  current  of  dry  air 
is  passing.  The  water  is  collected  and  weighed  in  a  U-tube 
containing  calcium  chloride.  The  combustion  tube  is  given  the 
same  form  as  that  usually  employed  in  the  determination  of 
carbon  and  hydrogen  in  organic  compounds.  It  is  also  cleansed, 
dried,  and  filled  in  the  same  manner  and  with  the  same  precau- 
tions as  when  an  organic  compound  is  to  be  analyzed.  The 
tube  should  have  an  internal  diameter  of  10  or  12  mm.,  a  length 
of  46  or  47  centimeters,  and  a  "bayonet"  with  a  horizontal 
length  of  12  centimeters.  There  are  introduced  (1)  a  roll  of 
copper  wire  gauze  10  mm.  in  length;  (2)  a  layer  of  coarse  lead 
chromate,  100  mm. ;  (3)  a  layer  of  fine  lead  chromate,  15  mm. ; 
(4)  the  mixture  containing  the  substance,  filling  from  65  to  70 
mm. ;  (5)  fine  lead  chromate,  50  mm. ;  (6)  coarse  chromate, 
180  mm. ;  (7)  a  roll  of  wire  gauze,  10  to  20  mm.  long.  After 
filling  the  tube  and  tapping  with  it  upon  the  table  to  produce  a 
channel  for  the  passage  of  air  arid  water  vapor,  it  is  placed  in  a 
combustion  furnace.  .  To  the  front  end  is  attached  an  apparatus 
consisting  of  a  weighed  U-tube  containing  calcium  chloride,  a 
soda-lime  tube,  and  a  small  U-tube  filled  with  pieces  of  potassium 
hydroxide.  The  bayonet  is  connected  with  an  air-drying  appa- 
ratus consisting  of  a  cylinder  filled  with  a  mixture  of  calcium 
*  Ber.  d.  d.  chem.  Ges.,  22,  221. 


SILICATES  307 

chloride  and  potassium  hydroxide,  a  cylinder  containing  strong 
sulphuric  acid,  and  a  third  cylinder  containing  a  solution  of  potas- 
sium hydroxide.  The  drying  apparatus  is  in  turn  connected  with 
an  air  gasometer.  The  remaining  steps  are  the  same  as  in  the 
combustion  of  an  organic  compound  in  a  current  of  air  or  oxygen. 

3.  Chatard's  Method.*    Only  the  main  features  of  this  pro- 
cess will  be  given  here.     For  the  details  the  original  article  must 
be  consulted,     A  platinum  boat  containing  the  silicate,  mixed 
with  an  alkaline  carbonate  if  necessary,  is  inclosed  in  a  cylinder 
of  platinum  foil,  to  prevent  loss  of  material  from  spattering,  and 
placed  in  a  platinum  combustion  tube  such  as  is  employed  in  the 
determination  of  carbon  in  iron  and  steel.     The  combustion  tube 
is  joined  at  one  end  with  an  air-drying  apparatus,  and  at  the  other 
with  a  weighed  absorption  apparatus  for  the  collection  of  water. 
To  avoid  the  error  arising  from  the  diffusion  of  hydrogen  through 
platinum,  the  tube  is  wrapped  with  several  layers  of  asbestus 
paper  which  has  been  soaked  in  a  saturated  solution  of  borax. 

4.  The  Lead  Oxide  Method,  which  in  some  respects  is  more 
simple  than  processes  1,  2,  and  3,  suffices  for  the  determination 
of  water  in  most  silicates,  and  has  the  additional  advantage  of 
leaving  the  material  in  good  condition  for  the  determination 
of  the  alkalies. 

5.  The  Borax  Method,  which  has  been  brought  to  a  high  state 
of  perfection  by  Jannasch.f 


EXERCISE  XXXIII 

THE  DETERMINATION  OF  WATER  IN  SILICATES  BY 
MEANS  OF  LEAD  OXIDE 

There  are  required : 

1.  A  hard  glass  tube  40  to  "45  centimeters  long,  with  an 
internal  diameter  of  15  or  16  mm.     One  end  of  the  tube  is 

*  Am.  Chem.  Jour.,  13,  110. 

t  Praktischer  Leitfaden  der  Gewichtsanalyse,  p.  243. 


308  QUANTITATIVE  EXERCISES 

drawn  out  so  that  a  small  rubber  tube  can  be  slipped  over  it. 
The  other  end  is  closed  with  a  perforated  rubber  stopper  through 
which  is  passed  a  short  piece  of  glass  tubing. 

2.  An  air-drying  apparatus  consisting  of  a  tube  filled  with 
soda-lime  and  a  tube  or  cylinder  containing  glass  beads  moist- 
ened with  concentrated  sulphuric  acid. 

3.  Weighed  absorption  tubes  for  the    collection  of   water. 
U -tubes  with  ground-glass  stoppers  are  recommended.     They 
are  to  be  filled  from  half  to  two-thirds  full  of  glass  beads  which 
are    afterwards   moistened   with    concentrated    sulphuric   acid. 
The  acid  should  be  introduced  with  a  pipette,  care  being  taken 
not  to  allow  any  of  it  to  come  in  contact  with  the  lubricant  used 
in  connection  with  the  stoppers,  and  the  quantity  of  acid  thus 
introduced  should  suffice  only  to  fill  the  horizontal  portion  of 
the  tubes.     Three  tubes  are  required. 

4.  A  quantity  of  pure  and  dry  lead  oxide.     This  is  prepared 
by  treating  a  solution  of  pure  lead  nitrate  with  ammonium  car- 
bonate, filtering,  washing  with  water  containing  a  little  ammo- 
nium carbonate  until  the  filtrate  gives  no  reaction  for  nitric 
acid,  and  heating  the  carbonate  in  a  platinum  dish  to  300°  or 
400°  on  a  sand  bath.     The  dish  must  not  be  heated  over  a  naked 
flame.     The  formation  of  some  red  oxide  during  the  decompo- 
sition of  the  carbonate  will  do  no  harm.     The  oxide  is  to  be 
tested  for  nitric  acid  and  kept  in  a  desiccator  until  required. 

Mix  in  a  platinum  crucible,  with  the  aid  of  a  platinum  wire 
or  spatula,  about  one  gram  of  finely  pulverized  tourmaline  which 
has  been  dried  in  a  desiccator  with  about  three  times  its  weight 
of  the  lead  oxide.  Transfer  the  mixture  to  a  roomy  platinum 
boat,  using  small  portions  of  the  oxide  to  cleanse  the  crucible. 
Place  the  combustion  tube  in  a  short  furnace.  Remove  the 
stopper  and  insert  the  boat.  Attach  the  drying  apparatus,  2,  to 
the  end  which  is  fitted  with  a  stopper,  placing  the  cylinder  con- 
taining the  beads  next  to  the  tube.  To  the  other  end  of  the 
combustion  tube  attach  an  un weighed  U-tube  containing  glass 
beads  moistened  with  sulphuric  acid,  bringing  the  glass  ends 


SILICATES  309 

close  together  within  the  rubber  connectors.  To  show  the  rapid- 
ity of  the  current,  insert  between  the  soda-lime  tube  and  the 
gasometer  a  small  U-tube  containing  a  little  mercury,  and  then 
pass  air  through  the  apparatus  for  half  an  hour  at  the  rate  of 
two  bubbles  per  second.  Now  insert  a  weighed  U-tube  between 
the  one  previously  used  and  the  combustion  tube,  and  conduct 
dried  air  through  the  apparatus  for  another  half  hour  at  the  same 
rate  as  before.  Remove  the  weighed  tube,  leaving  the  unweighed 
one  connected  with  the  combustion  tube,  and  weigh.  If  the  tube 
has  gained  sensibly  in  weight,  return  it  to  its  place,  add  another 
cylinder  containing  beads  moistened  with  sulphuric  acid  to  the 
arrangement  employed  to  dry  the  air,  and  pass  air  through  the 
apparatus  for  another  half  hour.  When  the  weight  of  the  U-tube 
is  found  to  be  sufficiently  constant,  insert  it  between  the  air- 
drying  apparatus  and  the  combustion  tube  and  attach  the  two 
remaining  weighed  U -tubes  to  the  other  end,  protecting  the  last 
one  from  the  moisture  of  the  air  by  joining  to  it  a  cylinder  or 
another  U-tube  containing  beads  and  sulphuric  acid.  Pass  air 
through  the  apparatus  and  heat  the  tube  in  the  vicinity  of  the 
boat,  raising  the  temperature  gradually,  until  the  glass  shows 
signs  of  softening.  Maintain  this  temperature  for  half  or  three- 
quarters  of  an  hour,  and  if  any  water  collects  in  the  front  part  of 
the  tube,  drive  it  into  the  absorption  apparatus  by  cautiously 
heating  the  places  upon  which  it  has  condensed.  Cool  the  tube 
slowly  and  gradually  without  interrupting  the  current  of  air,  and 
leave  it,  after  detaching  the  absorption  tubes,  fully  protected  from 
the  moisture  of  the  air.  Only  one  of  the  three  weighed  U-tubes, 
i.e.  that  next  to  the  exit  end  of  the  combustion  tube,  should  show 
any  increase  in  weight.  After  weighing,  return  the  absorption 
tubes  to  their  places  and  again  heat  the  silicate  as  before. 

The  presence  of  nitrate  in  the  lead  oxide  is  a  frequent  source 
of  error  in  this  method  of  determining  water  in  silicates.  It  is 
therefore  recommended  that  a  blank  determination  be  made 
with  a  few  grams  of  the  oxide  whenever  a  new  portion  of  this 
reagent  is  prepared. 


310  QUANTITATIVE  EXERCISES 


EXERCISE  XXXIV 

THE  DECOMPOSITION  OF  A  SILICATE  WITH  AN  ALKALINE 
CARBONATE  AND  THE  DETERMINATION  OF  THE  SILICA 

Mix  in  a  platinum  crucible,  with  the  aid  of  a  platinum  wire  or 
spatula,  or  a  glass  rod  the  ends  of  which  have  been  made  smooth 
by  softening  them  in  the  flame,  about  1  gram  of  finely  pulver- 
ized orthoclase  with  from  three  to  four  times  its  weight  of  dry 
potassium-sodium  carbonate  which  is  free  from  silica.  Pour  a 
little  of  the  carbonate  upon  a  glazed  paper,  cleanse  the  mixing  rod 
in  it,  and  then  transfer  the  material  on  the  paper  to  the  crucible. 
Cover  the  crucible  with  a  concave  lid,  place  it  in  a  clean  pipe- 
stem  triangle,  and  heat  for  twenty  minutes  in  the  flame  of  a  good 
Bunsen  burner.  Heat  afterwards  over  the  blast  lamp,  moder- 
ately for  a  time  and  later  more  strongly,  until  the  evolution  of 
carbon  dioxide  ceases  and  the  fused  material  becomes  trans- 
parent. Place  the  cooled  crucible  upon  its  side,  also  the  lid,  in 
a  porcelain  dish  of  convenient  size,  or  better  in  a  platinum  dish, 
and  add  a  little  water.  Cover  the  dish  with  a  watch  glass. 
Raise  one  side  of  the  'cover  from  time  to  time  and  introduce 
small  quantities  of  hydrochloric  acid  with  a  pipette  until  the 
evolution  of  carbon  dioxide  ceases.  Remove  the  cover,  the 
crucible,  and  its  lid,  washing  the  material  on  each  of  them  very 
carefully  back  into  the  dish.  Warm  the  dish,  with  the  cover  on, 
until  the  carbon  dioxide  in  solution  has  been  expelled,  and  then 
evaporate  to  dry  ness,  with  the  cover  off,  on  the  water  bath. 
Moisten  the  residue  with  strong  hydrochloric  acid  and  again 
evaporate  to  dryness.  If  the  material  balls  together,  pulverize  it 
with  a  platinum  spatula.  Repeat  the  evaporation  with  hydro- 
chloric acid  two  or  three  times  and  then  heat  the  dish  for  an  hour 
in  an  air  bath  at  a  temperature  of  110°.  Treat  the  residue  with 
a  little  strong  hydrochloric  acid,  add  water,  and  digest  for  half 
an  hour  on  the  water  bath.  Filter,  and  wash  the  silica  with 
hot  water  until  the  nitrate  yields  no  weighable  residue  on 


SILICATES  311 

evaporation.  It  is  very  difficult  to  distinguish  small  particles  of 
silica  upon  a  concave  surface  of  white  porcelain,  and  almost 
impossible,  therefore,  to  determine  whether  a  dish  of  this  kind 
has  been  sufficiently  cleansed.  For  this  reason  it  is  better  to 
use  a  platinum  dish  for  the  liberation  and  dehydration  of  the 
silicic  acid.  Partially  dry  the  silica  and  place  the  filter  contain- 
ing it,  folded  so  as  to  occupy  but  little  space,  in  a  weighed  plat- 
inum crucible.  Turn  the  crucible  on  its  side  in  a  triangle  and 
cautiously  burn  the  paper  to  a  white  ash.  Heat  the  silica  for 
a  time  over  a  Bunsen  burner,  and  then,  with  the  crucible  covered, 
to  constant  weight  over  a  blast  lamp. 

The  silica  must  be  tested  as  to  its  purity.  For  this  purpose 
treat  it  with  2  or  3  cc.  of  dilute  sulphuric  acid  and  8  or  10  cc. 
of  strong  hydrofluoric  acid,  which  leaves  no  weighable  residue 
when  evaporated  and  ignited  on  platinum.  Evaporate  to  dry- 
ness  on  a  sand  bath  or  iron  plate  and  ignite  over  a  Bunsen 
burner.  If  there  is  a  residue,  repeat  the  treatment  with  sul- 
phuric and  hydrofluoric  acids.  If  there  is  still  a  residue,  it 
consists  of  sulphates  or  oxides  of  metals  belonging  to  the  origi- 
nal silicate,  and  the  decomposition  of  the  mineral  was  incom- 
plete. If  the  bases  in  the  silicate  all  form  sulphates  which  are 
unstable  at  high  temperatures,  the  true  weight  of  the  silica  may 
still  be  found,  notwithstanding  the  incomplete  decomposition, 
by  subtracting  the  weight  of  the  highly  heated  residue  from 
that  of  the  material  before  treatment  with  hydrofluoric  acid.  If, 
on  the  other  hand,  a  part  of  the  bases,  like  the  alkalies,  form 
more  stable  sulphates,  the  experiment  must  be  repeated  with  a 
new  portion  of  the  mineral.  If  the  silicate  contains  titanic 
acid,  this  also  will  be  found  in  the  residue  after  treatment  of 
the  silica  with  hydrofluoric  and  sulphuric  acids. 

The  completeness  of  the  decomposition  of  a  silicate  may  also 
be  tested  by  heating  the  silica  for  an  hour  in  a  platinum  or 
silver  dish  with  a  solution  of  sodium  carbonate,  6  cc.  of  a  satu- 
rated solution  of  the  carbonate  and  12  cc.  of  water  for  each  0.1 
gram  of  the  silica.  If  there  is  an  insoluble  residue  after  such 


312  QUANTITATIVE   EXERCISES 

treatment,  this  should  be  fused  with  a  fresh  but  smaller  portion 
of  the  alkaline  carbonate. 

The  silica  which  is  liberated  from  its  salts  by  an  acid  is 
always  hydrated,  arid  in  that  condition  is  somewhat  soluble  in 
water.  The  evaporation  with  strong  hydrochloric  acid  and  the 
subsequent  heating  to  110°  are  for  the  purpose  of  dehydrating 
it  and  rendering  it  insoluble.  The  temperature  to  which  it  is 
heated  must  not,  however,  be  raised  much  above  110°  owing  to 
the  danger  of  recombination  with  the  bases.  The  silicates  so 
formed  would  doubtless  be  easily  decomposed  by  acid,  but  the 
liberated  silica  would  be  more  or  less  hydrated  and  therefore 
somewhat  soluble. 


OTHER  METHODS  OF  DECOMPOSING  SILICATES 
1.   DECOMPOSITION  BY  LEAD  OXIDES 

This  method  was  proposed  by  Berthier  in  1821*  and  again 
brought  forward  as  a  new  method  by  G.  Bong  in  1878. f  It 
appears,  however,  to  have  been  but  little  used.  The  general 
disinclination  to  employ  it  is  probably  due  to  the  common 
experience  that  platinum  vessels  when  heated  directly  in  a 
flame  are  often  injured  by  lead  compounds.  Berthier  proposed 
to  remove  this  objection  to  his  method  by  heating  with  a  mix- 
ture of  nitrate  and  carbonate  of  lead.  When  the  modification  of 
the  method  here  given  is  employed,  there  is  no  danger  of  injur- 
ing the  vessel  in  which  the  decomposition  is  effected,  provided 
the  silicate  contains  nothing  which  can  reduce  lead  compounds. 
If  the  mineral  contains  organic  matter  or  other  reducing  sub- 
stances, the  powder  must  be  ignited  in  the  air  before  mixing  it 
with  lead  oxide. 

The  pulverized  mineral  is  mixed  with  from  two  to  three 
times  its  weight  of  the  oxide,  and  the  mixture  transferred  to  a 

*  Annales  de  Chimie  et  de  Physique,  2  Ser.,  17,  28. 

t  Bulletin  de  la  Societt  Chimique,  Nouvelle  S^rie,  29,  50. 


SILICATES  313 

platinum  boat.  The  boat,  which  should  be  surrounded  with 
two  or  three  turns  of  platinum  wire  to  keep  it  away  from  the 
glass,  is  placed  in  a  combustion  tube  and  heated  in  a  short  fur- 
nace for  not  less  than  an  hour.  If  practicable,  the  temperature 
is  raised  to  the  fusing  point  of  the  mixture ;  if,  however,  it  is 
impossible  to  effect  a  fusion,  the  heating  is  continued  for  two 
hours.  The  boat  is  placed  in  a  platinum  dish  and  treated  with 
dilute  nitric  acid  and  enough  water  to  prevent  separation  of 
lead  nitrate.  A  gentle  heat  is  applied,  and  when  the  material 
separates  from  the  platinum  the  boat  is  removed  and  washed. 
After  complete  decomposition  by  the  nitric  acid  has  been  effected, 
the  contents  of  the  dish  are  evaporated  to  dryness ;  the  residue 
is  then  moistened  with  strong  nitric  acid  and  again  evaporated. 
The  evaporation  should  not  take  place  on  a  common  water  bath 
owing  to  the  danger  of  forming  lead  sulphate.  It  is  better  to 
heat  the  dish  on  a  large  sand  bath  or  on  an  iron  plate  until  but 
little  liquid  remains,  and  then  in  an  air  bath,  open  at  the  top, 
which  is  kept  at  a  temperature  below  110°.  The  residue  is 
finally  heated  for  an  hour  at  110°,  moistened  with  nitric  acid, 
treated  with  water,  and  again  heated  for  an  hour  nearly  to  the 
boiling  point  of  the  liquid.  The  silica,  after  collection  on  the 
filter,  is  washed  with  water  containing  a  very  little  nitric  acid 
and  determined  in  the  usual  manner.  The  atmosphere  of  the 
room  in  which  the  experiment  is  performed  must,  of  course,  be 
free  from  hydrogen  sulphide  and  the  vapors  of  all  other  sulphur 
compounds. 

The  method  is  an  excellent  one  for  the  decomposition  of  sili- 
cates, and  is  to  be  especially  recommended  for  those  in  which  it 
is  desired  to  determine  the  alkalies.  The  filtrate  from  the  silica 
contains  all  the  bases  in  the  form  of  nitrates,  and  these,  after 
the  precipitation  of  the  lead  by  hydrogen  sulphide,  may  be  sep- 
arated and  determined  by  -the  usual  methods. 


314  QUANTITATIVE  EXERCISES 

2.  DECOMPOSITION  BY  BISMUTH  OXIDE 

A  method  quite  similar  to  that  of  Berthier  was  proposed  by 
Hempel.*  It  consists  in  fusing  the  silicate  (0.5  gram)  with  a 
large  quantity  (10  grams)  of  bismuth  subnitrate  and  decompos- 
ing the  resulting  basic  silicate  with  strong  hydrochloric  acid. 
The  mixture  of  subnitrate  and  silicate  is  mildly  heated  as  long 
as  red  vapors  are  evolved,  and  then  for  ten  minutes  to  its  melt- 
ing point.  The  fused  mass  is  poured,  while  still  liquid,  into  a 
platinum  dish  floating  upon  cold  water.  Here  it  is  decomposed 
by  strong  hydrochloric  acid  and  the  silica  rendered  insoluble 
by  evaporation  to  dryness.  The  residue  is  treated  with  hydro- 
chloric acid,  in  which  everything  except  silica  is  soluble,  and  fil- 
tered. The  silica  is  washed  with  water  containing  hydrochloric 
acid  and  determined  in  the  usual  manner.  To  prepare  the  fil- 
trate for  a  determination  of  the  bases,  it  is  diluted  with  water 
until  the  greater  portion  of  the  bismuth  has  been  precipitated 
as  oxychloride.  The  precipitate  is  filtered  and  washed,  and  the 
remainder  of  the  bismuth  is  removed  by  treating  the  filtrate 
with  hydrogen  sulphide. 

3.  DECOMPOSITION  BY  HYDROCHLORIC  ACID 

Many  hydrated  silicates,  especially  those  belonging  to  the 
zeolite  section  and  some  anhydrous  ones,  are,  when  in  a  finely 
divided  condition,  completely  decomposed  in  open  vessels  by 
more  or  less  prolonged  treatment  with  concentrated  hydrochlo- 
ric acid.  And  many  silicates  which  are  ordinarily  but  slightly 
attacked  by  hydrochloric  acid  are  easily  decomposed  by  it  after 
long  ignition.  The  silicic  acid  which  is  liberated  by  the  action 
of  hydrochloric  acid  on  silicates  usually  presents  a  gelatinous 
and  semi-transparent  appearance ;  though  in  the  case  of  some 
species,  and  generally  when  the  mineral  has  previously  been 
dehydrated,  it  separates  as  a  white,  meal-like  powder.  A  few 
*  Zeitsch.  anal  Chem.,  20,  496. 


SILICATES  315 

easily  decomposed  silicates  yield,  when  treated  with  dilute  acid, 
a  silicic  acid  which  is  wholly  soluble  in  water.  Both  the  gelat- 
inous and  the  pulverulent  forms  of  silicic  acid  are  somewhat 
soluble  in  water  and  in  dilute  acids.  It  is  therefore  necessary 
in  all  cases,  after  decomposition  of  a  silicate  by  hydrochloric 
acid,  to  dehydrate  the  silica  by  repeated  evaporation  with  con- 
centrated acid  and  by  heating  to  110°.  The  testing  of  the 
silica  as  to  its  purity,  by  means  of  hydrofluoric  acid  or  sodium 
carbonate,  is  even  more  necessary  when  a  silicate  has  been  decom- 
posed by  an  acid  than  when  it  has  been  fused  with  the  alkaline 
carbonates  or  with  lead  oxide. 


4.  DECOMPOSITION  BY  ACIDS  UNDER  PRESSURE 

Silicates  which  in  open  vessels  are  but  slightly,  or  not  at  all, 
attacked  by  hydrochloric  and  sulphuric  acids  are  readily  decom- 
posed by  them  when  heated  under  pressure,  as  proposed  by 
Mitscherlich.*  About  1  gram  of  the  finely  pulverized  material 
is  placed  in  a  tube  of  hard  glass  which  has  been  closed  at  one 
end,  and  there  treated  with  an  excess  of  hydrochloric  acid  (25 
per  cent),  or  of  sulphuric  acid  (3  parts  of  the  concentrated  acid 
and  1  part  of  water).  The  tube  is  then  carefully  sealed  and 
heated  for  two  hours  in  an  iron  tube  at  a  temperature  of  200° 
or  210°.  The  tube  is  strongly  attacked  by  the  acids ;  hence  the 
method,  though  an  effective  one,  for  the  decomposition  of  sili- 
cates cannot  be  employed  when  the  substances  to  be  determined 
are  at  the  same  time  constituents  of  the  glass. 

This  difficulty  has  been  obviated  and  the  method  of  Mitscher- 
lich made  generally  applicable  by  Jannasch,f  who  places  the 
silicate  and  the  hydrochloric  acid  which  is  to  decompose  it  in  a 
platinum  tube  and  incloses  this  in  a  tube  of  glass.  The  plati- 
num tube  has  a  length  of  151  mm.  and  a  diameter  of  15  mm., 
and  is  provided  with  a  cap  through  which  passes  a  smaller  tube 

*  Journal  fiir  praktische  Chemie,  81,  108;  83,  455. 

t  Ber.  d.  d.  chem.  Ges.,  24,  273  ;  Zeitsch.  anal.  Chem.,  30,  334, 


316  QUANTITATIVE  EXERCISES 

having  a  length  of  32  mm.  and  a  diameter  of  5  mm.  The 
apparatus  has  a  capacity  of  26  cc.  and  weighs  57.5  grams.  The 
powdered  silicate  is  introduced  through  a  funnel  and  treated 
with  10  cc.  of  hydrochloric  acid  (4  parts  of  strong  acid  to  1 
part  of  water).  The  mineral  and  the  acid  are  thoroughly  mixed 
with  a  thick  platinum  wire,  the  end  of  which  is  bent  to  a  loop. 
On  removing  the  stirring  wire  it  is  rinsed  back  into  the  tube 
with  5  cc.  of  the  acid.  The  platinum  tube  is  closed  and  placed 
in  one  of  glass  which  has  an  internal  diameter  of  22  mm.  and  a 
wall  thickness  not  less  than  2  mm.  Acid  is  poured  into  the 
glass  tube  until  the  wider  portion  of  the  platinum  tube  is  half 
submerged.  The  open  end  of  the  glass  tube  is  drawn  out 
before  the  blast  lamp  to  a  small  diameter  and  the  air  in  the 
apparatus  expelled  by  inserting  a  narrow  tube  which  is  attached 
to  a  carbon  dioxide  generator.  The  tube  is  then  sealed  and 
heated  for  ten  or  twelve  hours  at  a  temperature  of  190°  or  210°. 
As  there  is  no  pressure  in  the  tube  when  cold,  the  precautions 
usually  observed  in  opening  sealed  tubes  are  unnecessary.  The 
contents  of  the  platinum  apparatus  are  transferred  to  a  platinum 
dish  and  treated  in  the  usual  manner.  Notwithstanding  the 
displacement  of  the  air  by  carbon  dioxide,  the  filtrate  from  the 
silica  is  generally  found  to  contain  a  small  quantity  of  platinum. 
This  is  to  be  removed  by  hydrogen  sulphide  before  proceeding 
to  a  determination  of  the  bases. 


5.  DECOMPOSITION  BY  HYDROFLUORIC  ACID 

A  method  very  generally  employed  when  the  alkalies  are  to 
be  determined  is  that  proposed  by  Berzelius.  It  consists  in  the 
decomposition  of  the  silicate  by  hydrofluoric  acid  and  the  con- 
version of  the  fluorides  of  the  bases  into  sulphates.  The  pul- 
verized material  is  treated  in  a  platinum  dish  with  a  small 
quantity  of  fuming  hydrofluoric  acid  and  stirred  with  a  plat- 
inum wire  after  each  addition  of  the  acid.  After  digesting  for 
some  time  on  a  water  bath  at  a  temperature  considerably  below 


SILICATES  317 

the  boiling  point,  a  quantity  of  sulphuric  acid  (1  part  of  con- 
centrated acid  and  1  part  of  water)  which  is  more  than  equiva- 
lent to  the  bases  of  the  "silicate  is  added.  The  contents  of  the 
dish  are  then  evaporated,  for  a  time  on  a  water  bath  and  later 
at  a  higher  temperature,  until  the  excess  of  the  sulphuric  acid 
has  been  expelled.  The  residue,  when  cold,  is  treated  with 
strong  hydrochloric  acid,  allowed  to  stand  for  some  hours,  and 
then  dissolved  in  water.  If  the  silicate  contains  no  bases  whose 
sulphates  are  insoluble,  and  the  decomposition  is  complete,  a 
clear  solution  will  be  obtained.  If  the  solution  is  not  clear, 
the  insoluble  matter  must  be  treated  with  hydrofluoric  and  sul- 
phuric acids  in  the  same  manner  as  the  original  material.  The 
solution  contains  all  the  bases  of  the  silicate  as  sulphates.  The 
silica  can,  of  course,  be  determined  only  indirectly.  The  hydro- 
fluoric acid  used  in  this  experiment  should  yield  no  nonvolatile 
residue  on  evaporation.  Owing  to  the  poisonous  nature  of  the 
vapors  of  hydrofluoric  acid,  the  whole  operation  should  be 
conducted  under  a  good  hood. 

There  are  various  modifications  of  Berzelius'  method :  (1)  the 
material  is  moistened  with  dilute  sulphuric  acid  and  exposed  for 
a  long  time  in  a  lead  box  to  the  vapors  of  hydrofluoric  acid  which 
are  generated  by  mixing  flu  or  spar  and  concentrated  sulphuric 
acid  in  the  bottom  of  the  vessel ;  (2)  the  mineral  is  heated  in 
a  platinum  tube  through  which  a  current  of  dry  gaseous  hydro- 
fluoric acid  is  passing ;  or  (3)  the  silicate  is  decomposed  by  heating 
it  with  ammonium  fluoride,  with  acid  potassium  fluoride,  or  with 
a  mixture  of  sodium  fluoride  and  acid  potassium  sulphate. 

A  method  of  decomposing  silicates  by  hydrofluoric  acid  for 
the  purpose  of  determining  ferrous  iron  will  be  given  in  a  later 
chapter. 

6.  DECOMPOSITION  BY  BARIUM  HYDROXIDE  AND  CARBONATE 

Formerly  silicates  were  often  decomposed  by  heating  them 
with  the  hydroxide  or  carbonate  of  barium.  The  first  com- 
pound attacks  platinum,  rendering  the  use  of  silver  crucibles 


318  QUANTITATIVE  EXERCISES 

necessary,  and  it  nearly  -always  contains  alkali,  while  the  second 
requires  for  its  effective  action  a  very  high  temperature,  —  a 
temperature  so  high,  in  fact,  that  the  alkalies  in  the  silicate  are 
likely  to  be  lost  by  volatilization.  Hence  these  methods,  though 
variously  modified  from  time  to  time  to  remedy  their  defects, 
have  been  almost  wholly  abandoned. 

Fusion  with  calcium  carbonate  has  also  been  recommended 
(Deville). 

7.  DECOMPOSITION  BY  AMMONIUM  CHLORIDE  AND  CALCIUM 
CARBONATE 

For  the  method  of  Lawrence  Smith,  see  Zeitschrift  fur  analy- 
tische  Chemie,  11,  85.  It  consists  in  heating  the  silicate  in  a 
platinum  crucible  of  special  form  with  a  mixture  of  calcium 
carbonate  and  ammonium  chloride.  The  process  is  a  simple 
one  and  in  the  case  of  many  silicates  is  quite  effective. 

EXERCISE  XXXV 
THE  DETERMINATION  OF  SILICON  IN  IRON 

The  treatment  of  the  iron  for  the  estimation  of  silicon  is 
precisely  the  same  as  for  the  estimation  of  phosphorus,  and  a 
single  portion  of  the  material  may,  therefore,  be  made  to  serve 
for  the  determination  of  both  elements. 

Proceed  as  directed  in  Exercise  XXX  until  the  residue  con- 
taining the  silica  has  been  collected  and  washed.  Dry  it,  burn 
the  paper,  and  mix  the  impure  silica  and  the  ash  in  a  platinum 
crucible  with  about  four  times  its  weight  of  potassium-sodium 
carbonate,  adding  a  little  potassium  nitrate.  Proceed  with  the 
mixture  as  directed  in  Exercise  XXXIV.  Having  found  the 
weight  of  the  silica  plus  that  of  any  impurities  it  may  contain, 
treat  with  hydrofluoric  and  sulphuric  acids  as  previously  directed 
and  deduct  the  weight  of  the  nonvolatile  residue.  The  silica 
derived  from  iron  often  contains  titanium  oxide,  hence  this 
correction  of  its  weight  should  never  be  omitted. 


SILICATES  319 

THE  SEPARATION  OF  SILICA  FROM  OTHER  SUBSTANCES 

By  the  fusion  of  silicates,  or  substances  containing  silica,  with 
the  alkaline  carbonates,  and  the  subsequent  treatment  of  the 
fused  mass  with  hydrochloric  or  nitric  acid,  etc.,  silica  is  quan- 
titatively separated  from  all  the  bases,  leaving  the  latter,  with 
the  exception  of  the  alkalies,  in  condition  to  be  separated  and 
determined  by  the  usual  methods. 

By  fusion  with  lead  oxide  and  subsequent  treatment  with 
nitric  acid,  etc.,  silica  is  separated  from  all  bases,  leaving  the 
alkalies,  as  well  as  all  the  other  bases  ordinarily  occurring  in 
silicates,  in  condition  to  be  determined.  The  presence  of  lead 
in  the  nitrate  containing  the  bases  of  the  silicate  does  not 
greatly  complicate  their  separation,  provided  none  of  them  are 
precipitated  from  acid  solutions  by  hydrogen  sulphide. 

Decomposition  with  hydrofluoric  and  sulphuric  acids  effects 
a  complete  separation  of  silica  from  all  metals. 

Decomposition  with  hydrochloric  acid,  when  complete,  to- 
gether with  the  evaporation  with  strong  hydrochloric  acid,  sep- 
arates silica  from  all  bases,  leaving  them  in  the  best  possible 
condition  for  determination. 

The  separation  of  silica  from  some  of  the  acid-forming 
elements  in  such  a  manner  that  the  latter  may  also  be  deter- 
mined is  more  difficult  than  its  separation  from  the  bases.  The 
substances  whose  presence  in  silicates  gives  rise  to  analyti- 
cal difficulties  which  render  necessary  some  modifications  of 
the  ordinary  methods  are  (1)  chlorine,  (2)  fluorine,  (3)  boron, 
(4)  sulphur,  (5)  titanium.  Owing  to  the  volatility  of  hydro- 
chloric, hydrofluoric,  and  boric  acids,  and  of  sulphur  com- 
pounds, excepting  sulphuric  acid,  a  part  or  the  whole  of  the 
first  four  of  these  elements  is  lost  in  rendering  the  silica  anhy- 
drous ;  while  the  fifth,  titanium,  always  remains  with  the  silica 
after  dehydration.  For  the  methods  which  are  to  be  followed 
in  the  analysis  of  silicates  containing  these  elements,  the 
handbook  of  Rose  should  be  consulted. 


CHAPTER  XIV 
THE  DETERMINATION  OF  CARBONIC  ACID 


EXERCISE  XXXVI 

I.  THE  VOLUMETRIC  DETERMINATION  OF  FREE  AND  SEMI- 
COMBINED  CARBONIC  ACID  IN  WATER 

PETTENKOFER'S   METHOD 

In  its  simplest  form  the  method  consists  in  the  addition  to 
the  water  of  a  measured  but  excessive  quantity  of  a  standard 
solution  of  barium  or  calcium  hydroxide,  and  the  determination 
of  the  excess  by  means  of  a  standard  solution  of  oxalic  acid. 
The  following  equations  will  illustrate  the  reactions  which  are 
expected  to  take  place  when  the  water  contains  only  free  car- 
bonic acid  and  acid  carbonates  of  the  alkaline  earths : 

H2CO3  +  Ca(OH)2  =  CaCO3  +  2  H2O, 
H2CO3CaC03  +  Ca(OH)2  =  2  CaCO3  +  2  H2O, 

Suppose,  however,  as  often  happens,  that  the  water  contains  acid 
carbonates  of  the  alkalies.  The  reaction  which  will  then  occur 
is  represented  by  the  equation 

2  KHC03  +  Ca(OH)2  =  CaCO3  +  2  H2O  +  K2CO3; 
or,  if  an  excess  of  the  hydroxide  is  added,  by  the  equation 
2  KHCO3  +  2  Ca(OH)2  =  2  CaC03+2  H2O  +  2  KOH. 

It  will  be  observed  that  the  alkalinity  of  the  water,  found 
by  subsequent  titration  with  an  acid,  is  the  same  whether  one 
or  two  molecules  of  the  hydroxide  are  added  for  each  molecule 
of  the  acid  carbonate  of  an  alkali.  It  will  also  be  noticed  that 

320 


THE  DETERMINATION  OF  CARBONIC   ACID          321 

the  semi-combined  carbonic  acid  is  correctly  determined  only 
when  for  every  molecule  of  an  acid  carbonate  of  an  alkali  not 
less  than  two  molecules  of  the  hydroxide  have  been  added. 
All  danger  of  error  from  this  source  is  avoided  by  adding  to 
the  water  or  to  the  standard  solution  of  hydroxide  a  quantity 
of  neutral  calcium  or  barium  chloride  ;  for 


There  is  also  avoided,  by  such  addition  of  a  chloride,  the  error 
resulting  from  the  presence  in  the  water  of  substances  which, 
like  the  soluble  sulphates,  precipitate  the  metals  of  the  alkaline 
earths.  A  further  advantage  to  be  gained  by  the  presence  of 
the  chloride  is  the  prevention  of  a  possible  reaction  between 
the  oxarlates  of  the  alkaline  metals  or  of  magnesium  on  the  one 
hand  and  calcium  carbonate  on  the  other,  by  which  calcium 
oxalate  and  the  carbonates  of  the  alkalies  and  of  magnesium 
are  formed. 

K2C2O4  +  CaCO3  =  CaC2O4  +  K2CO3, 
MgC2O4+  CaCO3  =  CaC2O4  +  MgCO3. 

These  carbonates,  if  formed,  would  neutralize  a  quantity  of  the 
standard  oxalic  acid  ;  in  other  words,  they  would  increase  the 
apparent  alkalinity  of  the  solution.  In  the  presence  of  a  chlo- 
ride of  an  alkaline  earth  no  such  irregularity  can  occur,  since 

K2C2O4  +  CaCl2  =  CaC2O4+2  KC1,  and 
MgC204  +  CaCl2  =  CaC204  +  MgCl2. 

If  the  water  contains  salts  of  magnesium,  ammonium  chlo- 
ride must  also  be  added  in  order  to  prevent  the  precipitation 
either  of  the  carbonate  or  of  the  hydroxide  of  that  metal.  The 
former  subsides  very  slowly,  while  the  precipitation  of  the  latter 
diminishes  the  alkalinity  of  the  solution. 

The  excess  of  the  calcium  hydroxide  cannot  be  immediately 
determined,  since  freshly  formed  calcium  carbonate  is  amor- 
phous and  somewhat  soluble  in  water,  imparting  to  the  latter 
an  alkaline  reaction.  On  standing,  the  amorphous  compound 


322  QUANTITATIVE  EXERCISES 

becomes  crystalline  and  insoluble.  The  transformation  is  has- 
tened by  heat.  The  application  of  heat  is,  however,  inadmis- 
sible when  the  solution  contains  ammonium  salts.  Freshly 
precipitated  barium  carbonate,  on  the  other  hand,  is  insoluble, 
but  any  advantage  which  might  on  this  account  be  expected 
from  the  use  of  a  standard  solution  of  barium  hydroxide,  rather 
than  one  of  calcium  hydroxide,  is  lost  when  the  water,  as  is 
usually  the  case,  contains  either  the  carbonate  or  the  sulphate 
of  calcium. 

There  are  required  : 

1.  A  saturated  solution  of  calcium  hydroxide,  which  is  pre- 
pared by  slaking  lime  from  marble,  agitating  the  product  with 
water,  and  allowing  the  suspended  matter  to  subside.     Filtra- 
tion should  not  be  attempted. 

2.  A  standard  solution  of  oxalic  acid  of  such  strength  that 
each  cubic  centimeter  is  equivalent  to  1  milligram  of  carbonic 
anhydride. 

3.  A  solution  of  calcium  chloride  (1  part  of  the  crystallized 
salt  in  5  parts  by  weight  of  water). 

4.  A  solution  of  ammonium  chloride  (1  part  of  the  salt  in 
8  parts  of  water). 

5.  A  sensitive  solution  of  litmus. 

Determine  how  much  of  the  standard  oxalic  acid  is  required 
to  neutralize  45  cc.  of  the  calcium  hydroxide  solution.  Place 
100  cc.  of  a  water  having  considerable  "  temporary  hardness  " 
(ordinary  hydrant  water  will  usually  suffice)  in  a  clean  and  dry 
flask.  Add  3  cc.  of  the  calcium  chloride,  2  cc.  of  the  ammonium 
chloride,  and  45  cc.  of  the  calcium  hydroxide  solution.  Close 
the  flask  with  a  rubber  stopper,  shake,  and  then  set  it  aside  for 
twelve  hours.  Withdraw  50  cc.  of  the  liquid  and  titrate  with  the 
standard  oxalic  acid,  using  litmus  as  the  indicator.  Repeat  the 
titration  with  a  second  50-cc.  portion  of  the  liquid.  Instead  of 
litmus,  turmeric  paper  may  be  employed  to  detect  the  neutral 
point,  in  which  case  the  acid  is  added  until  a  drop  of  the  liquid 
taken  out  upon  a  glass  rod  and  placed  upon  the  paper  no  longer 


THE  DETERMINATION  OF  CARBONIC  ACID  328 

produces  a  brown  ring.  If  the  water  contains  only  slight  quan- 
tities of  magnesium  salts,  ammonium  chloride  need  not  be  added. 
This  omission  would  be  of  advantage  because  in  the  absence  of 
ammonium  salts,  phenolphthalein,  which  is  much  more  sensitive 
than  litmus  or  turmeric,  could  be  used  as  the  indicator. 

The  volume  of  the  oxalic  acid  required  to  neutralize  50  cc.  of 
the  liquid  is  to  be  multiplied  by  3  and  the  product  subtracted 
from  the  volume  which  neutralizes  45  cc.  of  the  calcium  hydrox- 
ide solution.  The  difference  is  the  number  of  milligrams  of 
free  and  semi-combined  carbonic  anhydride  in  100  cc.  of  the 
water.  It  is  at  the  same  time  the  number  of  parts  by  weight  of 
the  anhydride  in  100,000  parts  of  the  water,  in  which  form  the 
quantitative  results  of  a  water  analysis  are  usually  stated. 

Another  100-cc.  portion  of  the  same  water  should  be  titrated 
with  a  standard  solution  of  hydrochloric  acid,  using  tropseolin 
or  methyl  orange  as  the  indicator.  The  acid  consumed  will  be 
equivalent  to  the  combined  carbonic  acid  if,  as  is  probably 
the  case,  the  water  contains  no  other  basic  substances  than 
carbonates. 

If  the  quantity  of  free  and  semi-combined  carbonic  acid 
exceeds  that  of  the  combined,  the  difference  between  the  two 
amounts  is  the  free  carbonic  acid  which  was  in  the  water. 

il.   THE   DETERMINATION  OF   TOTAL   CARBONIC    ACID 
IN  WATER 

Briefly  stated,  the  method  consists  in  (1)  the  precipitation  of 
the  carbonic  acid  by  calcium  hydroxide,  (2)  the  decomposition 
of  the  carbonate  by  hydrochloric  acid,  and  (3)  the  collection  of 
the  carbonic  acid  in  a  weighed  quantity  of  potassium  hydroxide. 

The  calcium  hydroxide  is  prepared  by  adding  water  to  freshly 
and  thoroughly  burned  lime  until  it  falls  into  a  fine  dry  powder. 
This  is  preserved  in  bottles  having  tightly  fitting  stoppers.  The 
hydroxide  always  contains  carbonate,  and,  since  this  cannot  be 
separated,  the  carbonic  acid  must  be  determined  in  a  weighed 


324  QUANTITATIVE  EXERCISES 

average  sample  of  the  material.  Owing,  however,  to  the  lack 
of  uniformity  in  the  distribution  of  the  carbonate  throughout 
the  hydroxide,  the  correction  of  the  results  by  means  of  such  a 
determination  is  a  matter  of  some  uncertainty.  It  is  for  this 
reason  that  the  use  of  any  unnecessary  excess  of  the  hydroxide 
is  avoided.  If  the  water  contains  alkaline  carbonates,  calcium 
chloride  must  also  be  added  in  order  to  insure  a  complete  pre- 
cipitation of  the  carbonic  acid.  The  slight  solubility  of  calcium 
carbonate  in  dilute  solutions  of  the  hydroxide  is  a  source  of 
some  error  in  the  method.  A  compensating,  and  probably  more 
than  equivalent,  error  results  from  the  unavoidable  exposure  to 
the  air  of  the  alkaline  material  during  nitration. 

The  determination  is  made  in  the  following  manner  :  A  flask 
holding  somewhat  more  'than  300  cc.  is  graduated  to  contain 
that  amount,  supplied  with  a  tightly  fitting  rubber  stopper, 
and  weighed.  There  is  then  introduced  a  quantity  of  calcium 
hydroxide  ranging  from  0.5  gram  to  3.0  grams,  according  to 
the  amount  of  carbonic  acid  to  be  determined,  after  which  the 
flask  is  closed  and  again  weighed.  The  flask  or  other  receptacle 
containing  the  water  under  investigation  is  placed  in  ice  water 
or  a  freezing  mixture  and  cooled  nearly  to  zero.  When  suffi- 
ciently cold,  the  water  is  transferred  by  means  of  a  siphon  to  the 
flask  containing  the  calcium  hydroxide.  The  quantity  so  trans- 
ferred is  ascertained  by  again  weighing  the  flask.  The  weigh- 
ing cannot,  however,  be  made  until  the  temperature  of  the 
water  has  risen  to  that  of  the  balance  room,  owing  to  the  con- 
densation of  atmospheric  moisture  upon  the  glass.  After 
weighing,  about  1  cc.  of  calcium  chloride  solution  (1  to  10)  is 
introduced.  The  flask  is  closed,  shaken,  and  then  heated  for 
three-quarters  of  an  hour  upon  a  water  bath  to  render  the  calcium 
carbonate  crystalline  and  less  soluble.  When  the  solid  matter 
has  sufficiently  subsided,  the  greater  portion  of  the  liquid  above 
it  is  quickly  filtered  through  a  paper,  with  the  least  possible 
disturbance  of  the  material  in  the  bottom  of  the  flask.  The 
filter  and  its  contents  are  then  returned,  without  washing,  to 


THE  DETERMINATION  OF  CARBONIC   ACID  325 

the  flask  which  still  retains  the  greater  portion  of  the  carbonate. 
The  remaining  steps  are  the  same  as  in  Exercise  XXXVII. 

If  the  water  can  be  taken  directly  from  a  large  supply,  as, 
for  instance,  from  a  spring,  it  is  better  to  proceed  in  the  follow- 
ing manner :  The  weighed  flask  containing  the  calcium  hydrox- 
ide is  provided  with  a  doubly  perforated  rubber  stopper  carrying 
two  small  glass  tubes  of  equal  length.  One  of  these  ends  just 
below  the  stopper,  and  the  other  near  the  lower  end  of  the 
neck.  The  upper  end  of  the  former  is  closed  with  the  finger 
and  the  flask  is  immersed  in  water.  On  removing  the  finger 
the  water  enters  the  flask  through  one  tube  while  the  air 
escapes  through  the  other. 

It  frequently  happens  that  the  carbonic  acid  in  bottled  waters 
which  have  been  saturated  with  the  gas  under  pressure  is  to  be 
determined.  The  plan  usually  adopted  in  such  cases  is  that 
proposed  by  Rochleder.*  A  cork  borer  is  pierced  with  a  small 
drill  at  a  distance  from  its  cutting  end  somewhat  greater  than  a 
cork's  length.  The  other  end  of  the  borer  is  closed  with  a 
stopper  in  which  there  is  a  glass  tube  bent  to  a  right  angle.  To 
this  is  attached  a  rubber  tube  which  is  supplied  with  a  screw 
pinchcock  to  regulate  the  flow  of  the  gas.  The  free  end  of  the 
rubber  tube  is  attached  to  th6  absorption  apparatus  described  in 
Exercise  XXXVII.  The  metallic  tube  is  then  bored  into  the 
stopper  by  turning  the  bottle  until  the  small  hole  appears  below 
the  cork.  When  the  gas  ceases  to  flow  a  second  and  smaller 
hole  is  bored  in  the  cork.  Through  this  is  pushed  a  glass  tube 
until  the  end  within  the  bottle  is  only  slightly  above  the  sur- 
face of  the  water.  Air  freed  from  carbonic  acid  is  then  aspi- 
rated for  a  few  minutes  through  the  whole  apparatus  in  order  to 
remove  the  gas  which  remains  in  the  bottle  above  the  water 
and  in  the  tubes  leading  to  the  absorption  apparatus.  If  the 
absorption  tubes  in  which  the  carbonic  acid  is  collected  are 
now  weighed,  the  quantity  of  gas  held  in  the  water  by  "  over- 
pressure" will  be  ascertained. 

*  Zeitsch.  anal.  Chem.,  1,  20. 


326  QUANTITATIVE  EXERCISES 

The  carbonic  acid  remaining  in  the  water  in  the  bottle  is 
determined  in  the  manner  already  described,  i.e.  a  siphon  is 
inserted  and  a  portion  of  the  water  transferred  to  a  weighed 
flask  containing  a  known  quantity  of  calcium  hydroxide,  etc. 

EXERCISE  XXXVII 

DETERMINATION  OF  CARBONIC  ACID  IN  CARBONATES 
I.   BY  ABSORPTION  IN  A  WEIGHED  QUANTITY  OF  SODA-LIME 

The  apparatus  required  consists  of : 

1.  A  flask  having  a  capacity  of  from  150  to  300  cc.     This  is 
provided  with  a  tightly  fitting,  doubly  perforated  rubber  stopper, 
having,  in  one  hole  a  small  dropping  funnel  with  a  glass  stopcock 
and  in  the  other  a  glass  tube  bent  to  a  right  angle.     The  stem 
of  the  funnel  reaches  nearly  to  the  bottom  of  the  flask,  while  the 
tube  ends  just  under  the  stopper.    The  tube  should  be  of  sufficient 
diameter  to  allow  the  water  which  condenses  in  it  to  return  to  the 
flask.     It  is  also  well  to  cut  or  grind  the  end  within  the  flask 
diagonally  across  the  tube  and  to  blow  a  bulb  in  the  vertical  limb 
just  above  the  stopper.     The  tube  serves  to  conduct  the  gas  from 
the  flask  into  the  absorption  apparatus.     The  funnel  is  employed 
for  the  introduction  of  the  hydrochloric  acid  which  is  to  decom- 
pose the  carbonate,  and  through  it  also  is  conducted  the  air  which 
is  to  displace  the  carbonic  acid  remaining  in  the  flask  when  the 
decomposition  is  finished.     It  is  closed  at  the  top  with  a  rubber 
stopper  carrying  a  glass  tube  bent  to  a  right  angle. 

2.  A  Geissler  absorption  apparatus  partly  filled  with  a  solu- 
tion of  potassium  hydroxide  (sp.  gr.  1.35).    This  is  attached  to 
the  glass  tube  at  the  top  of  the  dropping  funnel  by  means  of 
a  rubber  tube,  and  serves  to  remove  the  carbonic  acid  from  the 
air  which  is  aspirated  through  the  flask. 

3.  A  series  of  eight  connected   U-tubes  which,  for  conven- 
ience, are  designated  by  the  letters  a,  6,  <?,  c?,  e,  /,  g,  and  h  in 
the  order  of  their  distance  from  the  flask,  1.     The  tubes  a,  £>, 


THE  DETERMINATION  OF  CARBONIC  ACID  327 

and  c  have  a  height  of  170  mm.  and  an  internal  diameter  of 
16  mm.  The  tubes  d,  e,f,  g,  and  h  are  smaller,  having  a  height 
of  110mm.  and  a  diameter  of  12mm.  a,  5,  c,  c?,  g,  and  h 
are  closed  with  rubber  stoppers ;  while  e  and  /,  in  which  the 
carbonic  acid  is  absorbed  and  weighed,  are  closed  with  corks 
covered  with  sealing  wax.  It  is  better,  however,  to  use  for  e 
and  /  glass-stoppered  U -tubes  with  small  side  tubes,  which  are 
opened  and  closed  by  turning  the  stoppers.  The  whole  system 
of  tubes  is  suspended  at  a  convenient  height  above  the  working 
table  from  hooks  driven  into  a  horizontal  strip  of  wood.  The 
tube  a,  which  is  nearest  the  flask,  contains  just  enough  calcium 
chloride  to  fill  its  horizontal  portion;  b  is  filled  with  calcium 
chloride ;  and  <?,  with  small  pieces  of  pumice  stone  which  have 
been  soaked  in  a  boiling  concentrated  solution  of  copper  sulphate, 
and  then  dried  and  heated  to  a  temperature  sufficient  to  dehy- 
drate the  copper  salt.  Any  vapors  of  hydrochloric  acid  escap- 
ing from  the  flask  are  absorbed  in  c.  The  tube  d  is  filled  with 
calcium  chloride.  The  weighed  tubes  e  and  /are  filled  five-sixths 
full  of  coarsely  granulated  soda-lime  and  one-sixth  full  of  calcium 
chloride,  the  latter  being  placed  in  both  cases  in  the  limb  of  the 
tube  which  is  farthest  from  the  flask.  The  tube  g  serves  to  pro- 
tect e  and  /  from  the  moisture  and  carbonic  acid  of  the  air.  One 
limb,  that  toward/,  is  filled  with  calcium  chloride,  and  the  other 
with  soda-lime,  h  contains  a  little  water  to  indicate  the  rate  of 
the  flow  of  gas  through  the  apparatus.  The  rubber  connection 
between  h  and  g  is  provided  with  a  Mohr  pinchcock,  and  is  closed, 
whenever  practicable,  to  prevent  the  absorption  of  moisture  by 
the  soda-lime  in  the  outer  limb  of  g. 

The  calcium  chloride  in  the  tubes  #,  6,  and  d  must,  of  course, 
be  free  from  calcium  oxide  or  other  alkaline  substances.  Solu- 
tions of  the  fused  commercial  salt,  however,  always  exhibit  a 
strongly  alkaline  reaction.  Material  suitable  for  this  experi- 
ment may  be  prepared  by  adding  ammonium  chloride  to  a  solu- 
tion of  commercial  calcium  chloride,  evaporating,  and  heating 
until  the  ammonium  salt  has  been  expelled. 


$28  QUANTITATIVE  EXERCISES 

Weigh  about  half  a  gram  of  pulverized  and  dried  Iceland 
spar  or  other  pure  carbonate  into  the  flask  1  and  add  to  it  a 
little  water.  Weigh  the  absorption  tubes  e  and  /.  Connect 
the  different  parts  of  the  apparatus,  tying  with  shoemaker's 
waxed  thread  all  joints  made  with  rubber  tubing.  Close  the 
stopcock  in  the  dropping  funnel.  Open  the  pinchcock  between 
g  and  A,  and  attach  A  to  a  filter  pump.  If  the  apparatus  is 
tight,  the  air  will  soon  cease  to  bubble  through  the  water  in  A. 
Fill  the  dropping  funnel  with  dilute  hydrochloric  acid,  allow 
a  little  of  it  to  flow  into  the  flask  below,  and  disconnect  from 
the  filter  pump.  Introduce  other  small  quantities  of  acid  from 
time  to  time  until  the  mineral  is  dissolved.  Remove  with  a 
pipette  the  greater  portion  of  the  acid  remaining  in  the  funnel. 
Connect  the  funnel  with  the  Geissler  bulbs,  and  aspirate  air 
through  the  apparatus  at  a  moderate  rate,  heating  the  flask 
meanwhile  nearly  to  the  boiling  point.  Avoid  all  active  ebulli- 
tion in  order  to  prevent,  as  far  as  possible,  the  distillation  of 
water  into  the  U-tube  nearest  the  flask.  Continue  to  draw  air 
through  the  apparatus  until  the  weighed  tubes,  which  become 
hot  during  the  absorption  of  carbonic  acid,  have  been  quite  cold 
for  a  period  of  five  or  ten  minutes.  Remove  and  weigh  the 
tubes  e  and  /. 

For  a  second  determination  it  is  necessary  only  to  refill  a 
and  e,  and  to  exchange  the  places  of  e  and/. 

The  method  is  an  excellent  one  and  is  applicable  to  all 
carbonates. 


II.    THE  VOLUMETRIC  DETERMINATION  OF  CARBONIC  ACID 

Weigh  into  a  beaker  about  0.2  gram  of  ground  and  dried 
Iceland  spar,  add  a  little  water,  and  measure  in  about  twice  the 
quantity  of  standard  hydrochloric  acid,  which  is  required  to 
dissolve  the  mineral.  Cover  the  beaker  with  a  watch  glass. 
When  the  material  is  dissolved,  rinse  the  under  side  of  the 
covering  glass  back  into  the  beaker,  add  a  little  methyl  orange, 


THE  DETERMINATION  OF  CARBONIC    ACID  329 

and  neutralize  with  a  very  dilute  water  solution  of  potassium 
hydroxide  whose  concentration  need  not  be  known.  Having 
found  the  volume  of  the  alkali  required  to  neutralize  the  excess 
of  hydrochloric  acid,  measure  off  an  equal  portion  of  the  alkali 
and  titrate  it  with  the  standard  hydrochloric  acid,  using  methyl 
orange  as  the  indicator.  The  difference  between  the  volume 
of  the  acid  added  to  the  calcite  and  that  required  to  neutralize 
the  alkali  is  the  volume  of  hydrochloric  acid  required  to  decom- 
pose the  carbonate.  If  the  standard  acid  is  very  dilute,  e.g. 
one-tenth  normal,  the  solution  of  the  carbonate  may  be  has- 
tened by  gently  warming  the  liquid.  Its  temperature  should 
not,  however,  be  allowed  to  rise  above  50°  or  60°. 

This  method  is  applicable  to  all  substances  containing  car- 
bonates, provided  they  do  not  at  the  same  time  contain  oxides 
or  salts  (decomposed  by  hydrochloric  acid)  whose  acids  are 
not  sensitive  to  methyl  orange,  —  such  salts,  for  example,  as 
borates,  sulphides,  and  certain  silicates. 

There  are  many  other  methods  of  determining  carbonic  acid 
in  carbonates.  All  of  them  may,  however,  be  arranged  in  one 
or  another  of  the  three  following  groups. 

1.  The  carbonate  is  heated  with  a  weighed  quantity  of  some 
nonvolatile  substance  which  at  high  temperatures  displaces  the 
carbonic  acid,  —  such  substances,  for  example,  as  borax  glass, 
sodium  metaphosphate,  silica,  and  potassium  bichromate.     The 
acid  in  some  carbonates  may,  of  course,  be  expelled  by  heat 
alone. 

2.  The  carbonate  is  decomposed  in  a  weighed  vessel  by  an 
acid,  but  under  such  conditions  that  only  the  dried  carbonic 
acid  can  escape.     In  this,  as  in  the  preceding  case,  the  car- 
bonic acid  is  determined  by  loss. 

3.  The  gas  derived  from  the  carbonate  by  decomposing  it 
with  an  acid  is  collected  and  measured.     A  great  variety  of 
forms  of  apparatus  have  been  devised  and  recommended  for 
methods  2  and  3. 


330  QUANTITATIVE  EXERCISES 

THE  SEPARATION  OF  CARBONIC  FROM  OTHER  ACIDS 

The  substances  whose  presence  is  likely  to  give  rise  to  diffi- 
culties when  carbonic  acid  is  determined  by  the  method  employed 
in  Exercise  XXXVII  are  peroxides,  fluorides,  sulphides,  sul- 
phites, and  thiosulphates. 

1.  Peroxides  decompose  hydrochloric  acid  with  evolution  of 
chlorine.     Hence,  if  these  are  present  in  the  material  under  exam- 
ination, some  other  acid  (nitric  or  sulphuric)  should  be  used  to 
liberate  the  carbonic  acid.    If  the  material  also  contains  chlorides, 
the  carbonic  acid  gas  must  be  made  to  pass  through  a  solution 
of  silver  sulphate  before  entering  the  absorption  apparatus. 

2.  If  fluorides,  excepting  calcium  fluorides,  are  present,  the 
carbonate  should  be  decomposed  with  tartaric  or  citric  acid. 

3.  Hydrogen  sulphide  is  readily  absorbed  by  pumice  stone 
containing  anhydrous  copper  sulphate.     Hence  no  danger  is  to 
be  apprehended  from  the  presence  of  a  sulphide  in  the  carbon- 
ate unless  the  conditions  are  such  as  to  give  rise  to  the  forma- 
tion of  sulphur  dioxide.     In  that  case  the  sulphur  must  be 
oxidized  to  sulphuric  acid  by  the  addition  of  neutral  potassium 
chromate. 

4.  Sulphites  and  thiosulphates  also  are  to  be  converted  into 
sulphates  by  potassium  chromate  before  liberating  the  carbonic 
acid.     But  in  such  cases  sulphuric  or  nitric   acid  should  be 
employed  rather  than  hydrochloric  acid;  and  if  chlorides  are 
present,  the  gas   must   be   purified   by  passing   it   through  a 
solution  of  silver  sulphate. 

FILTRATION  IN  AN  ATMOSPHERE  FREE  FROM  CARBONIC  ACID 

When  solutions  of  the  caustic  alkalies  and  alkaline  earths 
are  filtered,  the  carbonic  acid  of  the  air  must  be  excluded. 
Figure  53, 1,  2,  and  3,  represents  convenient  and  simple  forms 
of  apparatus  for  such  exclusion.  They  may  also  be  employed  for 
filtration  in  an  atmosphere  of  nitrogen  or  of  hydrogen.  Tubes 


TlIK   DETERMINATION   OF  CAKIJOMC   ACIJ) 


331 


filled  with  solid  potassium  hydroxide  are  attached  by  means  of 
rubber  tubing  to  a,  b,  e,  and  c?,  and  each  connection  is  provided 
with  a  Mohr  pinchcock.  The  parts  marked  e  contain  the  niters. 
If  asbestus  is  to  be  used,  the  forms  of  e  represented  in  1  and  3 
are  employed.  If,  on  the  other  hand,  a  paper  filter  will  suffice, 
the  form  of  e  represented  in  2  is  to  be  used.  In  1,  e  consists 
of  the  simplest  form  of  calcium  chloride  tube.  The  filter  is 
made  in  the  same  manner  as  a  Gooch  filter,  upon  a  perforated 
porcelain  disk  placed  in  the  bottom.  In  2,  e  consists  of  two 
funnels,  one  inverted  over  the  other,  whose  edges  are  held 
together  by  a  rubber  band.  In  3  two  large  cylindrical  funnels, 
such  as  are  employed  when  filtering  with  a  Gooch  crucible,  are 


FIG.  53 

\ 

used.  Their  edges  are  also  held  together  by  a  rubber  band. 
The  filter  is  made  in  the  same  manner  as  for  e  in  1.  The  flask 
A  contains  the  liquid  to  be  filtered  and  B  receives  the  filtrate. 
These  flasks  are  not  represented  in  2  and  3.  Ordinarily  a  bottle 
rather  than  a  flask  is  employed  to  receive  the  filtered  liquid. 

The  filter  pump  is  attached  to  the  potassium  hydroxide  tube 
at  d  and  the  pinchcock  at  a  is  opened.  After  a  time  a  is  closed 
and  b  is  opened.  Finally,  the  pump  is  attached  to  c  and  a  is 
opened.  Having  thus  freed  the  whole  apparatus  from  carbonic 
acid,  the  pump  is  attached  to  d  and  c  is  opened.  Having  in 
this  way  filled  the  funnel  with  the  liquid  to  be  filtered,  c  is 
closed  and  a  is  opened.  To  refill  the  funnel  at  any  time  it  is 
necessary  only  to  close  a  and  open  c. 


332  QUANTITATIVE  EXERCISES 

If  it  is  desired  to  filter  in  an  atmosphere  of  nitrogen,  the 
potassium  hydroxide  tubes  at  a,  ft,  c,  and  d  are  exchanged  for 
absorption  apparatuses  containing  an  alkaline  solution  of  pyro- 
gallic  acid.  . 

To  filter  in  an  atmosphere  of  hydrogen,  the  stoppers  in  A  and 
e  are  supplied  with  small  vent  tubes,  drawn  out  to  fine  points, 
which  may  be  opened  and  closed ;  d  is  drawn  out  to  a  fine  point 
and  bent  downward  to  a  right  angle  to  dip  under  mercury; 
while  a  and  c  are  connected  each  to  a  limb  of  a  Y-tube.  The 
hydrogen  is  generated  in  a  Kipp's  apparatus,  and  enters  the 
filtering  apparatus  —  properly  purified  and  under  pressure  — 
through  the  third  limb  of  the  Y-tube.  The  remaining  steps 
are  as  follows:  (1)  a  and  b  are  opened;  (2)  b  is  closed  and  d 
is  opened;  (3)  d  is  closed  and  the  vent  in  A  is  opened;  (4)  a 
is  closed  and  c  is  opened;  (5)  the  vent  in  A  is  closed  and  that 
in  e  is  opened,  filling  the  funnel;  (6)  c  and  the  vent  in  e  are 
closed  and  a  is  opened.  The  pressure  of  the  hydrogen  upon 
the  liquid  in  the  funnel  forces  the  latter  through  the  filter.  To 
refill  the  funnel,  c  and  the  vent  in  e  are  opened  and  a  is  closed. 

DETERMINATION  OF  CARBONIC  ACID  IN  THE  AIR 

There  are  three  principal  methods  of  determining  atmos- 
pheric carbonic  acid,  each  of  which  has  been  variously  modified 
in  respect  to  details. 

1.  A  large  measured  volume  of  air  is  slowly  aspirated  through 
weighed  absorption  tubes.  The  arrangement  usually  employed 
for  this  purpose  is  that  proposed  by  Brunner,  which  permits  a 
simultaneous  determination  of  atmospheric  moisture.  It  con- 
sists of  a  large  gasometer  of  ascertained  capacity,  which  is  filled 
with  water  and  serves  as  an  aspirator,  and  a  series  of  six 
connected  U-tubes,  which  for  convenience  will  be  designated 
by  the  letters  #,  6,  <?,  c?,  e,  and  /.  The  tube  a  is  connected 
with  the  aspirator,  and  the  others  follow  in  alphabetical  order. 
The  tubes  a,  6,  e,  and  /  are  filled  with  small  pieces  of  pumice 


THE  DETERMINATION  OF  CARBONIC   ACID  333 

stone  which  have  been  moistened  with  concentrated  sulphuric 
acid,  while  the  intermediate  pair,  consisting  of  c  and  c?,  contain 
moistened  calcium  hydroxide.  All  the  tubes  except  a  are 
weighed.  The  air  is  deprived  of  its  water  while  passing  through 
/  and  e^  and  of  its  carbonic  acid  while  passing  through  d  and  c. 
The  tube  b  retains  any  water  which  may  have  been  acquired  by 
the  dry  air  while  passing  through  d  and  c.  The  increase  in  the 
weight  of  the  three  tubes  b,  c,  and  d  gives  the  carbonic  acid 
contained  in  the  volume  of  air  which  was  aspirated  through  the 
apparatus,  while  the  increase  in  the  weight  of  e  and/  gives  the 
amount  of  water  in  the  same. 

Some  of  the  details  requiring  special  attention  are  the  follow- 
ing :  (1)  Owing  to  the  permeability  of  rubber  to  moisture  and 
carbonic  acid,  the  glass  tube  ends  should  be  brought  close 
together  within  the  rubber  connectors,  and  the  latter  should  be 
painted  externally  with  shellac  for  some  distance  on  either  side 
of  the  glass  joint ;  (2)  the  pumice  stone  must  be  freed  from 
chlorine ;  for  this  purpose  it  is  thoroughly  moistened  with 
concentrated  sulphuric  acid  and  then  heated  to  redness  in  a 
Hessian  crucible. 

Since  the  quantity  of  carbonic  acid  contained  in  the  air  is 
very  minute  (under  normal  conditions  only  about  3  volumes  in 
10,000,  or  0.03  of  1  per  cent),  the  amount  of  air  which  must  be 
drawn  through  the  apparatus  is  usually  very  great.  If  the  air 
is  not  more  than  ordinarily  rich  in  carbonic  acid,  100  liters  of 
the  former  will  yield  about  30  cc.  of  the  latter,  and  the  increase 
in  the  weight  of  the  absorption  tubes  6,  e,  and  d  will  be  about 
60  milligrams. 

The  weight  of  water  obtained  from  a  given  volume  of  air  is, 
as  a  rule,  about  eight  times  as  great  as  that  of  the  carbonic  acid. 
It  is  to  be  remarked  that  if  air  is  dried  to  the  fullest  possible 
extent  by  concentrated  sulphuric  acid,  and  is  afterwards  passed 
through  weighed  tubes  containing  phosphorus  pentoxide,  an 
increase  in  the  weight  of  the  latter  is  observed,  showing  that 
sulphuric  acid  is  an  imperfect  dehydrating  agent  for  gases. 


334  QUANTITATIVE  EXERCISES 

2.  The  carbonic  acid  in  a  known  volume  of  air  is  absorbed 
by  a  measured  quantity  of  a  standard  solution  of  barium  hydrox- 
ide, and  the  excess  of  the  alkali  is  determined  by  means  of  a 
standard  solution  of  oxalic  acid.  This  is  Pettenkofer's  method. 

If  the  air  to  be  examined  is  rich  in  carbonic  acid,  the  deter- 
mination is  made  in  the  following  manner:  A  glass-stoppered 
flask  having  a  capacity  of  about  6  liters  is  carefully  calibrated 
and  filled  with  the  air  under  investigation.  Forty-five  cubic 
centimeters  of  the  standard  solution  of  barium  hydroxide  are 
introduced  and  the  flask  is  closed.  From  time  to  time,  for  an 
interval  of  half  an  hour,  the  flask  is  rotated  in  such  a  manner 
as  to  spread  the  solution  over  the  glass  surface  within.  After- 
wards the  liquid  is  quickly  poured  into  a  glass  cylinder  and  cov- 
ered to  protect  it  from  the  carbonic  acid  of  the  air.  Finally, 
when  the  precipitate  of  barium  carbonate  has  subsided,  a  meas- 
ured portion  of  the  clear  solution  is  withdrawn  by  means  of  a 
pipette  and  titrated  with  the  standard  acid. 

If  the  air  does  not  contain  more  than  the  normal  proportion  of 
carbonic  acid  (approximately  3  parts  in  10,000),  a  much  larger 
volume  of  it  should  be  treated.  It  is  then  advisable  to  employ 
some  efficient  form  of  gas-absorption  apparatus  for  the  standard 
barium  hydroxide,  and  a  large  aspirator  of  known  capacity.  Hav- 
ing removed  the  carbonic  acid  from  a  sufficiently  large  quantity 
of  air,  the  contents  of  the  absorption  tubes  must  be  transferred 
to  a  graduated  vessel  in  which  they  may  be  diluted  to  a  known 
volume.  This  is  made  necessary  by  the  evaporation  which  occurs 
while  the  air  is  being  aspirated  through  the  standard  solution. 

A  method  proposed  by  Mohf  differs  from  that  of  Pettenkofer 
in  that  the  barium  which  is  precipitated  as  carbonate  is  deter- 
mined instead  of  the  unneutralized  hydroxide.  For  this  pur- 
pose the  solution  of  hydroxide,  after  absorption  of  the  carbonic 
acid,  is  filtered  and  washed,  and  the  carbonate  remaining  on  the 
filter  is  dissolved  in  hydrochloric  acid.  The  barium  may  then 
be  weighed  as  chloride  after  evaporation  of  the  solution,  or  as 
sulphate  after  precipitation  by  sulphuric  acid. 


THE  DETERMINATION  OF  CARBONIC  ACID  335 

One  source  of  error  in  the  method  of  Mohr  has  been  found 
in  the  fact  that  a  paper  through  which  the  hydroxide  has  been 
filtered  retains  barium  even  though  the  filter  was  washed  with 
water  until  the  filtrate  gave  no  reaction  for  the  metal.  The 
magnitude  of  the  error  and  therefore  the  amount  of  the  correc- 
tion to  be  applied  may  be  ascertained  by  filtering  a  solution  of 
equal  concentration  through  a  paper  of  the  same  size  as  that  to 
be  used  in  the  experiment,  afterwards  extracting  with  hydro- 
chloric acid,  and  determining  the  weight  of  the  barium  which 
has  been  retained. 

A  somewhat  similar  difficulty  has  been  observed  in  the  case 
of  all  methods  which  involve  the  use  of  a  standard  solution 
of  barium  hydroxide.  In  contact  with  glass  a  portion  of  the 
metal  is  rendered  insoluble  and  the  concentration  of  the  solu- 
tion thereby  diminished.  Errors  from  this  cause  are  easily 
avoided  by  first  washing  with  portions  of  a  standard  solution 
of  barium  hydroxide  all  glass  surfaces  with  which  it  is  to  come 
in  contact. 

A  standard  solution  of  barium  hydroxide  which  is  to  be  used 
with  one  of  oxalic  acid  must  not  contain  even  traces  of  the 
caustic  alkalies.  The  reason  for  this  exclusion  will  appear  if 
we  consider  the  following  equations  which  represent  the  reac- 
tions which  take  place  when  oxalic  acid  is  added  to  a  solution 
containing  a  caustic  alkali,  barium  hydroxide,  and  suspended 
barium  carbonate : 

(1)  2KOH-fH2C204-K2C204+2H20. 

(2)  KaC204+BaC08  = 

(3)  K2CO3  +  H2C2O4  = 

(4)  H2C03  +  Ba(OH)2  = 

The  potassium  oxalate  and  barium  carbonate  which  are  formed 
as  represented  in  equations  (3)  and  (4)  again  react  as  indicated 
in  equation  (2),  etc.  The  barium  carbonate  which  is  necessary 
for  the  institution  of  this  recurring  series  of  reactions  is  never 
wanting  in  unfiltered  solutions  of  the  hydroxide,  and  it  is 


336  QUANTITATIVE  EXERCISES 

nearly  always  formed  in  small  quantities  during  a  titration,  in 
consequence  of  unavoidable  exposure  of  the  solution  to  the  air. 
To  ascertain  whether  a  given  solution  of  barium  hydroxide 
contains  a  caustic  alkali,  it  is  necessary  only  to  filter  it  out  of 
contact  with  the  air,  to  measure  off  two  equal  portions  of  the 
filtrate,  to  add  to  one  of  these  a  small  quantity  of  pure  barium 
carbonate,  and  finally  to  titrate  both  portions  with  oxalic  acid. 
If  the  portion  to  which  the  carbonate  was  added  requires  more 
acid  for  its  neutralization  than  the  other,  a  caustic  alkali  is 
present.  A  solution  of  barium  hydroxide  containing  a  caustic 
alkali  may  be  rendered  fit  for  use  with  oxalic  acid  by  the 
addition  of  barium  chloride : 

2  KOH  +  BaCl2  =  2  KC1  +  Ba(OH)2. 

It  is  hardly  necessary  to  mention  the  fact  that  in  computing 
the  proportion  of  carbonic  acid,  the  temperature,  pressure,  and 
the  hygroscopic  condition  under  which  the  air  was  measured 
must  be  taken  into  account. 

3.  A  measured  portion  of  air  is  deprived  first  of  its  moisture, 
and  second  of  its  carbonic  acid  by  absorption,  and  the  contrac- 
tion which  takes  place  in  each  operation  is  determined  in  a 
graduated  and  calibrated  apparatus  which  permits  the  correct 
reading  of  minute  differences  of  volume.  The  best  apparatus 
yet  devised  for  this  method,  whether  for  the  determination  of 
carbonic  acid  or  of  atmospheric  moisture,  is  that  of  Pettersson.* 


THE  PREPARATION  OF  CARBONIC  ACID  GAS  FREE  FROM  AIR 

The  usual  methods  of  preparing  carbonic  acid  gas  for  labora- 
tory purposes  are : 

(1)  By  heating  a  carbonate  —  acid  sodium  carbonate,  magne- 
site,  manganese  carbonate,  or  a  mixture  of  sodium  carbonate  and 
potassium  bichromate  —  and  (2)  by  decomposing  a  carbonate  with 
some  acid — acid  sodium  carbonate,  marble,  or  magnesite. 
*  Zeitsch.  anal  Che'm.,  25,  467. 


THE  DETERMINATION  OF  CARBONIC  ACID  337 

Both  of  these  methods,  as  they  are  ordinarily  employed, 
yield  a  product  which  is  contaminated  with  air.  The  reasons 
for  this  are  sufficiently  obvious.  In  the  first  place,  it  is  diffi- 
cult wholly  to  displace  the  air  within  the  vessels  in  which  the 
carbonic  acid  is  generated.  Much  may  be  accomplished,  how- 
ever, by  giving  to  the  vessels  a  rational  form,  i.e.  a  form  which 
will  facilitate  the  expulsion  of  the  air  by  the  carbonic  acid.  In 
general,  it  may  be  said  that  they  should  be  made  as  narrow  in 
every  part  as  is  practicable,  in  order  that  the  current  of  gas 
moving  toward  the  exit  may  be  as  rapid  as  possible ;  also  that 
they  should  have  no  "dead  spaces,"  i.e.  spaces  where  the  gas 
may  come  to  a  partial  rest.  Moreover,  the  arrangement  of  parts 
should  be  such  that  the  direction  of  the  current  is  generally 
upward  rather  than  downward. 

Again,  if  acids  are  used  to  decompose  the  carbonate,  the 
absorbed  air  becomes  a  source  of  contamination  which  is  usu- 
ally very  persistent.  This  difficulty  may  be  partially  remedied 
by  boiling,  provided  the  acid  is  a  nonvolatile  one  or  does  not 
thereby  become  too  dilute.  If  the  acid  cannot  be  boiled  and 
the  absorbed  air  must  be  expelled  by  displacement,  a  narrow 
form  of  generator  is  especially  advantageous. 

But  the  most  serious  difficulty  presents  itself  when  compact 
minerals,  like  marble  and  magnesite,  especially  the  former,  are 
employed  as  the  source  of  the  carbonic  acid.  These  always  con- 
tain air,  which  is  slowly  liberated  during  the  entire  time  that  the 
mineral  is  undergoing  decomposition  either  by  heat  or  by  an  acid. 
In  the  case  of  marble  it  has  been  proposed  to  extract  the  included 
air  by  placing  the  fragments  of  the  mineral  under  water,  which 
is  afterwards  boiled  for  a  long  time ;  also  by  placing  the  wet 
material  under  the  bell  of  an  air  pump  and  exhausting.  Neither 
of  these  remedies  is  efficacious  though  both  are  of  some  value. 
It  is  better  in  all  cases,  if  practicable,  to  reduce  the  carbonate 
which  is  to  serve  as  the  source  of  the  gas  to  a  fine  powder. 

By  careful  attention  to  the  form  of  the  generating  apparatus 
and  its  connections,  and  to  the  condition  of  the  materials,  it  is 


338  QUANTITATIVE  EXERCISES 

possible  to  diminish  the  proportion  of  air  in  the  gas  to  limits 
which,  for  most  purposes,  are  tolerable.  If  a  purer  gas  is 
required,  the  following  method  is  recommended. 

A  hard  glass  tube  somewhat  longer  than  a  combustion  fur- 
nace is  closed  at  one  end  and  nearly  filled  with  finely  pow- 
dered and  dried  magnesite.  The  open  end  is  then  drawn  out 
and  attached  to  a  Sprengel  pump.  The  tube  is  exhausted  and 
then  heated  throughout  its  whole  length  until  the  pressure 
within  is  restored.  It  is  then  allowed  to  cool  and  to  remain 
for  some  time  undisturbed.  Afterwards  it  is  again  exhausted. 
The  gas  which  will  be  obtained  on  reheating  the  magnesite 
will  be  quite  free  from  air. 

If  still  greater  precautions  are  thought  to  be  necessary,  it  is 
well  to  reheat,  cool,  and  again  exhaust  the  tube. 

The  determination  of  carbonic  acid  in  other  gas  mixtures 
than  air  is  considered  in  a  later  chapter. 


CHAPTER  XV 

THE  DETERMINATION   OF  CARBON  AND  HYDROGEN  IN 
ORGANIC  COMPOUNDS 

THE  DRYING  OF  GASES 

As  a  rule,  the  hydrogen  and  carbon  of  organic  compounds  are 
oxidized,  the  former  to  water  and  the  latter  to  carbon  dioxide, 
and  the  products  of  the  oxidation  are  collected  in  weighed 
absorption  tubes.  The  absorbents  employed  for  carbon  dioxide 
are  either  soda-lime  or  potassium  hydroxide,  usually  the  latter. 
Both  are  efficient  and  no  extraordinary  precautions  are  neces- 
sary in  their  use.  The  proper  management  of  the  materials 
employed  for  the  collection  of  the  water  —  calcium  chloride, 
sulphuric  acid,  phosphorus  pentoxide,  etc.  —  is,  on  the  other 
hand,  a  matter  of  some  difficulty  and  requires  a  knowledge  of 
the  peculiarities  of  each. 

1.  CALCIUM  CHLORIDE 

The  salt  is  used  in  two  forms,  as  "  granular  "  and  as  "  fused  " 
calcium  chloride.  The  first  is  obtained  by  evaporating  a  solu- 
tion of  the  compound  to  dryness  and  heating  the  residue  for 
a  long  time  at  about  200°.  The  product  still  contains  from 
'20  to  26  per  cent  of  water,  or  approximately  "two  molecules. 
The  fused  material  is  anhydrous  except  in  so  far  as  it  has 
acquired  water  in  consequence  of  exposure.  The  anhydrous  or 
fused  form  absorbs  water  with  somewhat  greater  avidity  than 
the  granular,  though  the  difference  between  them  in  this 
respect  is  not  great  at  low  temperatures.  Solutions  of  both 
the  fused  and  the  granular  salt  exhibit  an  alkaline  reaction 

339 


340  QUANTITATIVE  EXERCISES 

owing  to  the  presence  of  calcium  hydroxide  derived  from  the 
oxide  which  is  formed  during  the  drying  of  the  chloride : 

CaCL  +  KLO  -  2  HC1  +  CaO. 


If  the  chloride  is  to  be  used  for  the  determination  of  water  in 
a  gas  which  also  contains  carbon  dioxide,  the  presence  in  it 
of  calcium  oxide  cannot,  of  course,  be  tolerated.  It  is  often 
attempted  to  escape  the  mischievous  effect  of  the  presence  of 
the  oxide  by  passing  a  current  of  dry  carbon  dioxide  or  of  dry 
hydrochloric  acid  gas  through  the  tube  containing  the  chloride, 
before  using  the  same  for  the  absorption  of  water.  It  has  been 
shown,  however,  that  the  conversion  of  the  oxide  into  carbonate 
or  chloride  by  this  means  is  only  superficial,  and  that  the  mate- 
rial is  therefore  again  in  condition  to  absorb  carbon  dioxide 
whenever  its  outer  surface  is  moistened  with  water. 

The  granular  form  may,  however,  be  obtained  in  a  neutral 
condition,  i.e.  free  from  oxide,  by  evaporating  a  solution  of  the 
chloride  with  ammonium  chloride  and  heating  the  residue  until 
the  latter  salt  has  been  expelled. 

Calcium  chloride,  whether  granular  or  fused,  is  an  imperfect 
drying  agent.  This  has  been  repeatedly  proved  by  passing  a 
moist  gas  through  a  series  of  weighed  tubes  containing  the 
chloride  and  then  through  other  weighed  tubes  containing 
strong  sulphuric  acid.  It  has  been  uniformly  found  that 
though  the  weight  of  the  last  calcium  chloride  tube  in  the 
series  remained  unchanged,  the  first  sulphuric  acid  tube  became 
heavier.  Furthermore  it  was  shown  by  Dibbits  that  when  satu- 
rated air  is  dried  to  the  fullest  possible  extent  by  calcium  chlo- 
ride containing  26.22  per  cent  of  water  or  2.19  molecules,  the 
gas  still  retains  per  liter  the  following  quantities  of  water:  0.3, 
1.0,  or  3.3  milligrams,  according  as  the  desiccation  is  effected 
at  0°,  15°,  or  30°.  These  quantities  correspond  respectively  to 
6,  8,  and  11  per  cent  of  the  total  amount  of  water  originally 
in  the  saturated  air. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS     341 

That  the  efficiency  of  calcium  chloride  as  a  drying  agent  is 
greatly  modified  by  temperatures  was  also  shown  by  Dibbits  in 
the  following  manner : 

Air  dried  by  calcium  chloride  at  the  temperature  of  the  room 
was  passed  through  a  number  of  other  weighed  calcium  chloride 
tubes.  The  two  end  tubes  of  the  series  remained  at  the  tem- 
perature of  the  room,  while  those  which  intervened  were  main- 
tained at  other  temperatures,  some  above  and  some  below  that 
of  the  room.  The  results  of  an  experiment  of  this  kind  are 
given  below  in  tabular  form.  The  letters  in  the  first  vertical 
column,  from  a  to  e,  designate  the  tubes  in  their  order,  the 
dried  air  entering  a  and  then  passing  through  5,  c,  etc.  The 
second  column  gives  the  temperatures  of  the  various  tubes  dur- 
ing the  passage  of  the  air,  and  the  third  their  gain  or  loss  in 
weight. 

TEMPERATURE  OF  THE  ROOM  16.9° 

TUBES  TEMPERATURE  CHANGE  IN  WEIGHT 

a 16.9° 0.0000  gram 

b 0.0° +  0.0452     " 

c 30.2° -  0.1565     « 

d 0.0° +  0.1559     « 

e  . 16.9° -  0.0442     " 

Total     ....  -f  0.0004     « 

It  appears  from  the  foregoing  table  that  if  air  which  has  been 
dried  by  calcium  chloride  at  one  temperature  is  passed  over 
another  portion  of  the  salt  having  a  different  temperature,  it 
(the  air)  will  gain  or  lose  water  according  as  the  temperature  of 
the  second  portion  of  calcium  chloride  is  higher  or  lower  than 
that  of  the  material  by  which  it  was  first  dried.  Stated  in 
another  way,  the  weight  of  the  individual  absorption  tubes  in  a 
series  through  which  previously  dried  air  is  passing  may  be 
increased  or  diminished  at  will  by  simply  lowering  or  raising 
their  temperature.  If  the  tubes  by  which  the  gas  enters  and 
leaves  the  series  have  the  same  temperature  as  the  calcium 


342  QUANTITATIVE  EXERCISES 

chloride  by  which  the  air  was  first  dried,  the  algebraic  sum  of 
the  gain  and  loss  will  be  zero  within  the  limits  of  necessary 
errors  in  the  weighing  of  such  apparatus. 

The  important  bearing  of  these  facts  on  the  determination  of 
hydrogen  in  organic  compounds,  as  well  as  on  all  other  quantita- 
tive operations  in  which  water  is  collected  in  a  similar  manner, 
will  appear  later. 

2.  SULPHURIC  ACID 

As  stated  already,  concentrated  sulphuric  acid  is  a  better  dry- 
ing agent  for  gases  than  calcium  chloride.  The  quantities  of 
water  which  were  found  by  Dibbits  to  remain  in  air  after  dry- 
ing with  calcium  chloride  are  the  quantities  which  are  absorbed 
by  sulphuric  acid  after  the  air  has  been  dried  to  the  fullest  pos- 
sible extent  by  calcium  chloride. 

Moreover,  the  efficiency  of  sulphuric  acid  is  much  less  influ- 
enced by  temperature.  It  desiccates  nearly  equally  well  at 
all  temperatures  between  -  30°  and  -f  25°.  Above  30°  its  effi- 
ciency declines,  but  not  rapidly. 

It  has  been  stated  that  when  a  gas  containing  carbon  dioxide 
is  conducted  over  or  through  sulphuric  acid,  a  sensible  quantity 
of  this  constituent  is  retained  by  the  acid.  It  appears,  however, 
that  all  of  the  carbon  dioxide  so  retained  is  easily  removed  by 
passing  through  the  acid  another  gas,  e.g.  air,  which  is  free  from 
the  dioxide. 

A  certain  degree  of  volatility  at  ordinary  temperatures  has 
also  been  ascribed  to  the  acid,  but  Morley  has  shown  that  a  liter 
of  air  which  has  passed  through  pure  sulphuric  acid  of  1.84 
specific  gravity,  at  .ordinary  temperatures,  cannot  contain  more 
than  from  0.002  to  0.003  milligram  of  sulphuric  anhydride  (SO3). 
If  the  presence  of  pyrosulphuric  acid,  from  which  the  tri- 
oxide  does  volatilize,  is  to  be  apprehended,  the  acid  may  be 
diluted  with  6  or  8  per  cent  of  water,  which  will  not  materially 
diminish  its  efficiency  as  a  desiccating  agent. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS  343 

Sulphuric  acid  often  contains  sulphur  dioxide,  but  this  may 
be  removed  by  boiling  or  by  passing  air  through  it. 

The  best  form  of  absorption  tube  for  sulphuric  acid,  as  well 
as  for  most  other  absorbents,  is  usually  the  glass-stoppered 
U-tube  with  small  side  tubes  —  for  the  entrance  and  exit  of  the 
gas — which  are  opened  or  closed  by  turning  the  stoppers.  These 
are  filled  with  fragments  of  broken  glass  or  with  beads,  which 
are  then  thoroughly  moistened  with  the  a,cid.  The  quantity  of 
the  acid  is  so  regulated  that  the  portion  which  collects  in  the 
bottom  shall  not  quite  fill  the  horizontal  part  of  the  tube.  The 
glass  can  be  remoistened  at  any  time  by  turning  the  tube  first 
in  one  direction  and  then  in  another. 

3.  PHOSPHORUS  PENTOXIDB 

The  most  energetic  of  all  absorbents  for  water  is  phosphoric 
anhydride.  It  is,  however,  but  little  used  except  in  cases  where 
the  extreme  desiccation  of  a  gas  is  necessary.  The  objection  to 
its  more  frequent  employment  is  the  exceedingly  viscous  liquid 
which  is  formed  wherever  water  comes  in  contact  with  the  oxide. 
This  liquid  soon  clogs  a  tube  filled  in  the  ordinary  manner  and 
prevents  the  passage  of  gas.  The  difficulty  is  partially  reme- 
died by  loosely  filling  the  tube  with  glass  wool  and  then  dis- 
tributing the  pentoxide  through  it  in  such  a  manner  that  no 
large  quantity  of  the  absorbent  is  collected  in  any  one  place, 
especially  in  the  end  of  the  tube  through  which  the  gas  enters. 
Usually  a  straight  tube  only  partially  filled  with  the  pentoxide 
is  employed.  A  channel  for  the  passage  of  the  gas  can  then 
always  be  secured  by  giving  to  the  tube  a  horizontal  position 
and  tapping  until  the  material  distributes  itself  along  the  bottom. 
Whenever  such  a  course  is  practicable,  a  gas  which  is  to  be 
dried  by  phosphorus  pentoxide  should  first  be  made  to  pass 
through  strong  sulphuric  acid.  It  will  then  enter  the  pentox- 
ide in  a  nearly  dry  condition,  and  the  formation  of  any  consid- 
erable quantity  of  the  viscous  liquid  will  be  avoided.  The 


344  QUANTITATIVE  EXERCISES 

degree  of  desiccation  will  be  the  same,  of  course,  as  if  the  anhy- 
dride alone  had  been  used. 

Commercial  phosphorus  pentoxide  sometimes  contains  the 
trioxide.  This  may  be  removed  by  resublimation  in  a  current 
of  dry  air. 

If  air  dried  to  the  fullest  possible  extent  by  sulphuric  acid  is 
passed  through  weighed  phosphorus  pentoxide  tubes,  the  latter 
increase  slightly  in  weight.  The  increase  is,  however,  by  no 
means  so  great  as  that  observed  when  air  dried  by  calcium 
chloride  is  made  to  pass  through  sulphuric  acid.  According  to 
Dibbits  it  amounts  to  about  0.002  milligram  per  liter  of  air, 
and  according  to  Morley*  it  is  precisely  this  quantity  of  water 
which  strong  sulphuric  acid  is  unable  to  remove  from  the  air. 

The  question  whether  a  gas  which  has  been  dried  by  phos- 
phorus pentoxide  still  contains  water  has  been  investigated  in 
a  very  ingenious  manner  by  Morley.  Air  dried  by  phosphorus 
pentoxide  was  drawn,  with  the  aid  of  a  vacuum  pump,  through 
a  glass  apparatus  consisting  of  one  piece  but  having  in  reality 
two  parts.  Part  I,  that  first  entered  by  the  gas,  contained 
moistened  calcium  chloride,  and  Part  II,  phosphorus  pentoxide. 
The  gas,  in  passing  from  I  to  II,  was  made  to  traverse  an 
exceedingly  fine  capillary  tube,  and  it  therefore  entered  the  sec- 
ond compartment  in  a  greatly  expanded  condition.  If  now  the 
apparatus  should  be  found  to  lose  weight,  the  loss  must  be  due 
to  the  inability  of  the  oxide  in  II  completely  to  desiccate  the 
air  with  its  diminished  tension  of  water  vapor.  A  minute  loss 
in  weight  was  observed,  amounting  to  about  0.25  milligram  of 
water  in  10,000  liters  of  gas.  As  estimated  by  the  experi- 
menter, air  passing  through  the  apparatus  at  the  rate  of  three 
liters  per  hour  would  occasion  in  ten  years  a  loss  of  weight 
amounting  to  about  10  milligrams. 

*  Zeitsch.  anal.  Chem.,  24,  541. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS  345 

4.  ANHYDROUS  COPPER  SULPHATE 

According  to  Fresenius,  anhydrous  copper  sulphate  is  some- 
what less  hygroscopic  than  calcium  chloride.  The  method  by 
which  it  is  usually  prepared  for  use  in  drying  a  gas  is  that  pro- 
posed by  Stolba.  Fragments  of  pumice  stone  are  placed  in  a 
concentrated  solution  of  copper  sulphate  and  the  liquid  is  boiled 
until  the  air  has  all  been  expelled  from  the  stone.  The  pieces 
are  then  drained,  dried,  and  heated  to  the  temperature  required 
to  dehydrate  the  sulphate  (about  200°). 

5.  CALCIUM  OXIDE 

Quicklime,  even  though  freshly  heated,  is  markedly  less  effi- 
cient as  a  drying  agent  than  any  of  the  substances  hitherto 
mentioned.  According  to  Van  der  Pleats  it  leaves  twice  as 
much  water  in  a  gas  as  calcium  chloride. 

6.  POTASSIUM  HYDROXIDE 

Calcium  oxide  and  fused  potassium  hydroxide  are  employed 
to  dry  ammonia  gas.  The  latter  substance  is  far  more  efficient 
for  this  purpose  than  the  former.  As  a  desiccating  agent  for 
gases  not  containing  carbon  dioxide  it  is  said  to  excel  calcium 
chloride  (Van  der  Plaats). 

Fresenius  arranges  the  absorbents  investigated  by  him  in  the 
following  order,  according  to  their  efficiency : 

1.  Quicklime. 

2.  Anhydrous  copper  sulphate. 

3.  Calcium  chloride  (fused  and  granular). 

4.  Concentrated  sulphuric  acid. 

5.  Phosphorus  pentoxide. 

In  nearly  all  quantitative  operations  in  which  water  is 
absorbed  and  weighed  dry  air  or  some  other  gas  must  be 


346  QUANTITATIVE  EXERCISES 

employed  to  sweep  the  water  into  the  absorption  apparatus,  and 
it  is  in  drying  the  air  so  employed  that  the  most  serious  mis- 
takes are  likely  to  be  made.  On  the  basis  of  what  has  already 
been  said  regarding  the  various  absorbents,  the  following  rule 
may  be  formulated :  The  same  kind  of  material  should  be  used 
to  dry  the  air  and  to  absorb  the  water ;  in  other  words,  the  mate- 
rial employed  for  the  one  purpose  should  be  just  as  efficient 
as  and  no  more  so  than  that  used  for  the  other.  Suppose,  for 
example,  that  an  organic  compound  has  been  burned  by  copper 
oxide  in  a  glass  tube  in  the  usual  manner,  and  that  the  water  is 
to  be  collected  and  weighed  in  a  calcium  chloride  tube ;  the  cur- 
rent of  air  which  is  introduced  into  the  combustion  tube  for  the 
purpose  of  carrying  the  water  into  the  absorption  apparatus  must 
also  be  dried  by  calcium  chloride.  Moreover,  the  two  drying 
apparatuses  must  be  maintained  at  nearly  the  same  temperature. 
If  the  air  were  dried  by  sulphuric  acid  and  the  water  absorbed 
by  calcium  chloride,  the  result  would  be  too  low ;  while  if  the 
reverse  arrangement  were  adopted,  it  would  be  too  high. 

Rubber  stoppers  and  tubes  are  always  a  source  of  embar- 
rassment in  the  determination  of  gas  constituents.  They  are 
somewhat  permeable  to  water,  carbon  dioxide,  etc.,  and  certain 
precautions  are  therefore  necessary  in  their  use.  A  tube  which 
is  to  be  weighed  should  not  be  closed  with  a  rubber  stopper 
but  rather  with  a  good  cork  whose  projecting  end  is  smoothly 
covered  with  sealing  wax.  It  is  better,  of  course,  whenever 
practicable,  to  use  for  absorbents  tubes  which  are  closed  by 
accurately  ground  glass  stoppers.  If  the  different  parts  of  an 
absorption  apparatus  must  be  joined  by  means  of  rubber  tubes, 
the  glass  ends  of  the  adjacent  parts  should  be  brought  as  close 
together  as  possible.  The  rubber  connectors  should  be  tied 
with  shoemaker's  waxed  thread,  and  the  space  between  the  ties 
should  be  painted  with  shellac.  The  tubing  which  is  to  be  used 
in  making  such  connections  should  be  first  dried  by  placing  it, 
while  hot,  in  a  desiccator  over  strong  sulphuric  acid,  and  all 
pieces  of  rubber  so  employed  should  be  kept  in  the  desiccator 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS   347 

when  not  in  use.  All  rubber  connectors  should  be  removed 
before  weighing  an  absorption  apparatus,  and  the  latter,  when 
out  of  use,  should  be  kept  in  a  desiccator  with  some  of  the  same 
material  with  which  it  is  filled. 

In  order  to  avoid  any  error  which  might  arise  in  a  determina- 
tion of  water  from  a  hydrated  condition  of  the  glass,  the  tube 
in  which  the  water  is  to  be  formed  or  liberated  should  be  sub- 
jected to  a  preliminary  heating,  and  while  hot  there  should  be 
conducted  through  it  a  current  of  dry  air.  If  practicable,  the 
temperature  to  which  it  is  raised  should  be  higher  than  that  to 
which  it  will  be  subjected  in  the  actual  determination. 

The  articles  relating  to  the  desiccation  of  gases,  which  may 
be  read  with  advantage,  are  indicated  below.* 

MATERIALS    EMPLOYED   IN   THE   DETERMINATION  OF 
CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS 

1.  COPPER  OXIDE 

The  oxide  which  is  sold  under  the  designation  "  for  organic 
analysis,"  if  it  has  not  been  previously  used,  is  generally  suffi- 
ciently free  from  objectionable  impurities.  Such  material  usually 
requires  only  to  be  heated  in  an  atmosphere  of  air  or  oxygen  — 
in  order  to  oxidize  any  metallic  copper  or  organic  matter  which 
it  may  contain  and  to  expel  water  —  and  to  be  thereafter  pro- 
tected from  moisture  and  dust.  A  hard  glass  tube,  with  an 
internal  diameter  of  15  or  16  mm.  and  somewhat  longer  than 
a  combustion  furnace,  is  drawn  out  at  one  end  to  permit 
the  attachment  of  a  small  rubber  tube.  A  plug  of  ignited 
asbestus  or  a  short  roll  of  clean  copper  wire  gauze  is  placed  in 
the  end  which  was  drawn  out,  and  the  remainder  of  the  tube  is 
nearly  filled  with  the  copper  oxide  to  be  heated.  The  tube, 

*Fresenius,  Zeitsch.  anal.  Chem.,  4,  177  ;  Dibbits,  Zeitsch.  anal  Chem.,  15, 
121 ;  Morley,  Zeitsch.  anal.  Chem.,  24,  533  ;  27, 1 ;  Van  der  Plaats,  Eecueil  des 
Travaux  Chimiques,  etc.,  6,  45. 


348  QUANTITATIVE  EXERCISES 

with  an  efficient  drying  arrangement  attached  to  each  end, — 
calcium  chloride  or  concentrated  sulphuric  acid,  —  is  placed  in  a 
combustion  furnace,  and  the  oxide  is  heated  for  a  long  time  to 
low  redness  in  a  current  of  air  or  of  oxygen  which  is  aspirated 
or  forced  by  pressure  through  the  apparatus.  The  tube  is  after- 
wards very  slowly  and  uniformly  cooled  and  is  left  with  the 
drying  apparatus  attached  until  the  oxide  is  required  for  use. 
Since  two  grades  of  copper  oxide,  "  coarse  "  and  "  fine,"  are  used 
in  every  combustion,  it  is  necessary  to  prepare  a  quantity  of  both 
kinds ;  and  for  this  purpose  it  is  more  convenient  to  employ  two 
tubes,  each  of  which  contains  only  one  grade  of  the  oxide. 

Another  method  of  preparing  the  oxide,  which  is  often  used, 
is  as  follows :  The  oxide  is  placed  in  a  Hessian  crucible,  with 
the  addition  of  a  small  quantity  of  nitric  acid,  if  it  contains 
much  metallic  copper,  and  is  heated  in  a  furnace  to  redness, 
but  not  sufficiently  high  to  cause  the  formation  of  a  copper 
silicate.  The  crucible  is  removed  from  the  furnace,  and  the 
oxide,  while  still  very  hot,  is  transferred  to  a  flask  which  is 
afterwards  closed  with  a  calcium  chloride  tube  as  soon  as  the 
temperature  of  its  neck  will  permit.  A  pear-shaped  flask  of 
uniform  thickness  is  usually  employed,  and  to  avoid  the  danger 
of  cracking,  it  is  heated  in  a  sand  bath  before  introducing  the 
hot  oxide.  The  crucible  is  grasped  with  a  pair  of  tongs  and  the 
oxide  poured  into  the  flask  through  a  copper  funnel. 

The  copper  oxide  which  is  used  in  organic  analysis  should  be 
free  from  (1)  the  alkalies,  (2)  the  alkaline  earths,  and  (3)  the 
chlorides  of  copper.  The  first  two  lose  or  recover  carbon  diox- 
ide according  to  the  temperature  of  the  copper  oxide,  while  the 
chlorides  of  copper  are  decomposed  by  hot  moist  air  with  libera- 
tion of  hydrochloric  acid.  The  presence  of  the  alkalies  may  be 
detected  by  boiling  a  portion  of  the  oxide  in  a  platinum  or 
porcelain  dish  with  water  of  neutral  reaction,  filtering,  and 
treating  the  filtrate  with  phenolphthalein  or  litmus.  If  an 
alkali  is  found,  the  whole  body  of  the  oxide  should  be  boiled 
with  water,  filtered,  and  thoroughly  washed  with  hot  water. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS   349 

Calcium,  which  is  sometimes  present  in  the  form  of  carbonate, 
may  be  found  by  digesting  a  small  quantity  of  the  oxide  with 
warm  acetic  acid,  precipitating  the  copper  by  hydrogen  sulphide, 
filtering,  and  treating  the  filtrate  with  ammonium  oxalate.  To 
remove  the  carbonate,  the  oxide  must  be  digested  for  a  long 
time  with  water  containing  a  little  nitric  acid,  filtered,  and 
thoroughly  washed.  Copper  oxide  which  has  been  treated  for 
the  removal  of  alkalies  or  of  calcium  carbonate  must,  after 
drying,  be  heated  by  one  of  the  methods  previously  given,  or 
in  a  muffle.  Copper  oxide  containing  chloride  is  purified  by 
heating  it  in  a  glass  tube  through  which  a  current  of  moist  air 
is  passing.  When  the  escaping  vapors  no  longer  exhibit  an 
acid  reaction,  a  current  of  dry  air  is  substituted. 

2.  LEAD  CHROMATE 

Lead  chromate  is  used  instead  of  copper  oxide  when  the  sub- 
stance to  be  analyzed  contains  sulphur,  or  a  halogen,  or  is  diffi- 
cult to  burn.  It  is  prepared  in  the  following  manner:  A  solution 
of  lead  acetate  ("  sugar  of  lead")  is  slightly  acidified  with  acetic 
acid  and  treated  with  a  small  excess  of  potassium  bichromate. 
The  precipitate  of  lead  chromate  is  collected  upon  a  cotton 
cloth  which  is  stretched  on  a  wooden  frame,  and  washed  with 
water  until  the  nitrate  is  found  to  be  free  from  potassium.  The 
material  is  then  dried,  fused  in  a  Hessian  crucible,  and  poured, 
while  in  a  molten  condition,  upon  a  cold  stone  or  iron  plate. 
The  cooled  mass  is  broken  up  —  for  the  most  part  pulverized  — 
and  sifted  in  order  to  separate  the  fine  from  the  coarse  material. 
Finally,  the  various  grades  are  dried  at  a  temperature  somewhat 
above  100°  in  flasks  or  tubes  which  are  afterwards  provided 
with  calcium  chloride  tubes  to  protect  the  chromate  from  the 
moisture  of  the  air. 

A  given  volume  of  lead  chromate  contains  one  and  one-half 
times  as  much  oxygen  as  an  equal  volume  of  copper  oxide.  At 
a  red  heat  it  fuses,  and  at  higher  temperatures  it  gives  up  a 


350  QUANTITATIVE  EXERCISES 

portion  of  its  oxygen.  The  product  of  this  decomposition  is  a 
mixture  of  chromic  oxide  and  basic  lead  chromate.  When  only 
moderately  heated  it  oxidizes  less  energetically  than  copper 
oxide.  Its  greater  efficiency  at  higher  temperatures  is  due  to 
its  fused  condition  and  to  the  liberation  of  oxygen. 

It  has  frequently  been  noticed  that  a  sample  of  the  chromate 
which  had  been  prepared  for  use  in  organic  analysis  gave  up,  on 
being  fused,  a  quantity  of  carbon  dioxide.  It  has  not,  however, 
been  ascertained  whether  the  gas  so  liberated  was  absorbed 
while  the  chromate  was,  at  some  previous  time,  in  the  molten 
condition,  or  whether  it  resulted  from  the  oxidation  of  organic 
matter  with  which  the  material  had  become  contaminated. 

Lead  chromate  may  be  used  for  three  or  four  combustions 
without  reoxidation,  but  if  longer  service  is  required  of  it,  it 
should  be  moistened  with  nitric  acid,  dried,  fused,  etc.  If  occa- 
sionally treated  in  this  manner,  the  material  may  be  used 
indefinitely. 

Formerly  lead  chromate  was  supposed  to  be  less  hygroscopic 
than  copper  oxide,  and  its  use  was  therefore  recommended  in 
cases  in  which  an  exact  determination  of  hydrogen  was  required. 
This  opinion  appears,  however,  to  have  been  erroneous. 

The  rather  easy  fusibility  of  lead  chromate  is  an  objection  to 
its  use  except  in  the  case  of  those  substances  which  are  the 
most  difficult  to  burn.  R.  de  Roode*  has  overcome  this  difficulty 
in  a  very  satisfactory  manner  by  mixing  with  the  powdered 
chromate  one-fourth  its  weight  of  red  oxide  of  lead.  The  mix- 
ture is  washed  in  a  funnel  with  water  and  drawn,  with  the  aid 
of  the  filter  pump,  into  a  compact  mass.  While  still  moist  the 
material  is  broken  up  with  a  spatula  into  pieces  about  the  size 
of  a  pea,  which  are  heated  to  redness,  a  small  portion  at  a  time, 
in  a  porcelain  crucible.  When  cold,  the  pieces  are  still  further 
broken  up  in  a  porcelain  mortar  into  pieces  about  the  size  of 
wheat  grains,  and  the  finer  material  is  separated  from  the  coarser 
by  sifting.  This  mixture  does  not  liquefy  at  a  red  heat. 
*  Am.  Chem.  Jour.,  12,  226. 


CARBON   AND  HYDROGEN  IN  ORGANIC  COMPOUNDS      351 

3.  METALLIC  COPPER 

When  an  organic  compound  containing  nitrogen  is  burned 
with  copper  oxide  or  lead  chromate,  more  or  less  of  the  nitro- 
gen is  converted  into  nitric  oxide.  The  formation  of  this  com- 
pound takes  place  even  though  the  nitrogen  in  the  substance 
burned  is  not  in  direct  combination  with  oxygen.  The  nitric 
oxide  thus  formed,  if  not  destroyed,  would  be  converted  by 
oxygen  and  water  into  nitric  and  nitrous  acids,  and  these 
would  be  absorbed  in  the  calcium  chloride  and  potassium 
hydroxide  which  are  employed  to  collect  the  water  and  carbon 
dioxide.  In  practice  it  is  always  destroyed  by  reduction  to 
nitrogen,  and  the  reducing  agent  is  always  metallic  copper. 
The  choice  of  metals  available  for  this  purpose  is  exceedingly 
limited,  owing  to  the  fact  that  most  of  the  metals  which  are 
capable  of  depriving  nitric  oxide  of  its  oxygen  also  reduce 
carbon  dioxide. 

Copper,  in  contact  with  moist  air,  becomes  tarnished  with 
oxide,  and  in  this  condition  it  is  unfit  for  the  reduction  of 
nitric  oxide.  It  is  therefore  necessary  first  to  subject  the  metal 
itself  to  the  action  of  some  reducing  agent  in  order  to  give  to 
it  a  fresh  metallic  surface.  The  most  effective  agent  for  this 
purpose  is  hydrogen,  but  unfortunately  copper,  when  heated  in 
an  atmosphere  of  hydrogen,  occludes  a  considerable  quantity 
of  the  gas  and  does  not  readily  give  it  up  again.  The  copper 
may  also  be,  and  usually  is,  reduced  by  the  vapors  of  some 
inflammable  liquid  of  low  boiling  point,  like  methyl  alcohol. 

Either  of  the  following  methods  will  yield  a  product  suitable 
for  the  reduction  of  nitric  oxide. 

1.  A  hard  glass  tube  is  compactly  filled  with  granular  copper 
oxide  and  heated  to  dull  .redness  in  a  combustion  furnace. 
Hydrogen  gas  —  which  has  been  purified  by  passing  through  an 
acid  and  afterwards  an  alkaline  solution  of  potassium  perman- 
ganate —  is  conducted  over  the  hot  oxide.  When  the  reduction 
is  complete,  the  current  of  hydrogen  is  exchanged  for  one  of  air, 


352  QUANTITATIVE  EXERCISES 

which  is  likewise  made  to  pass  through  the  permanganate  solu- 
tions and  also  through  a  calcium  chloride  tube.  The  oxygen 
of  the  air  is  absorbed  on  entering  the  tube,  and  a  portion  of  the 
copper  —  that  nearest  the  entrance  —  is  converted  into  oxide. 
The  remainder  of  the  copper  is  surrounded  by  an  atmosphere  of 
nitrogen,  into  which  the  occluded  hydrogen  diffuses  and  by 
which  it  is  carried  out  of  the  tube.  The  temperature  of  the 
metallic  copper  should  be  raised,  while  the  nitrogen  is  passing 
over  it,  somewhat  higher  than  that  to  which  the  oxide  was 
heated  while  undergoing  reduction,  and  the  current  should  be 
continued  until  a  layer  of  oxide  75  or  100  mm.  in  length  has 
been  formed.  Afterwards  the  tube  is  slowly  cooled  —  first 
the  portion  containing  the  metal  and  lastly  that  containing  the 
oxide  —  and  the  copper  is  transferred  to  a  convenient  vessel  and 
placed  in  a  calcium  chloride  desiccator.  According  to  Hempel,* 
copper  prepared  in  the  manner  described  above  contains  no 
hydrogen. 

2.  Compact  rolls  of  closely  woven  copper  wire  gauze  —  having 
a  diameter  slightly  less  than  the  internal  diameter  of  the  com- 
bustion tube  in  which  they  are  to  be  used  —  are  dropped,  while 
red  hot,  into  a  glass  tube  250  mm.  in  length,  in  the  bottom 
of  which  are  a  few  drops  of  methyl  alcohol,  and  the  tube  is 
closed  with  a  cork  to  prevent  reoxidation.  When  cold  the 
roll  is  transferred  to  an  air  bath  and  heated  for  some  time  at 
a  temperature  of  140°.  According  to  G.  Neumann,  f  copper 
reduced  by  methyl  alcohol  yields,  when  burned  in  air  or  oxygen, 
small  quantities  of  water  and  carbon  dioxide.  It  has  also  been 
proposed  by  Th.  Weyl  J  to  prepare  metallic  copper  for  organic 
analysis  by  passing  vapors  of  formic  acid  over  the  hot  oxide. 

It  appears  that  the  hydrogen  occluded  by  copper  is  capable 
of  reducing  carbon  dioxide  to  the  monoxide,  thereby  affecting 
the  determination  of  carbon  as  well  as  that  of  hydrogen. 

*  Zeitsch.  anal  Chem.,  17,  414.          t  Ibid.,  32,  99.          J  Ibid.,  21,  559,. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS   353 

4.  OXYGEN 

The  oxygen  required  to  complete  the  combustion  of  organic 
compounds  is  best  prepared  by  heating  the  usual  mixture  of 
finely  powdered  potassium  chlorate  (20  parts)  and  pyrolusite 
(1  part).  The  mixture  is  placed  in  a  glass  or  metallic  retort  — 
having  a  capacity  not  less  than  twice  the  volume  of  the  powder 
and  provided  at  its  outlet  with  a  rather  wide  rubber  tube  to  con- 
duct the  oxygen  into  the  gasometer  —  and  is  heated,  gently  at 
first  and  afterwards  more  vigorously,  with  a  triple  burner  which 
is  held  in  the  hand.  When  the  mass  begins  to  melt,  the  retort 
is  gently  shaken  in  order  to  effect  a  uniform  distribution  of  the 
heat;  and  when  the  evolution  of  gas  becomes  quite  strong,  the 
end  of  the  rubber  tube  is  inserted  in  the  gasometer.  The  com- 
mercial "compressed"  oxygen  usually  contains  hydrocarbons 
and  cannot  be  used  in  the  combustion  of  organic  compounds. 

One  gram  of  potassium  chlorate  yields  about  2.7  liters  of 
oxygen.  The  gas  contains  chlorine  and  usually  carbon  dioxide, 
both  of  which,  however,  are  removed  while  the  oxygen  is  pass- 
ing through  the  apparatus  described  under  5. 

5.  AN  APPARATUS  FOR  THE  PURIFICATION  OF  OXYGEN 

AND  AIR 

The  air  and  oxygen  employed  in  the  determination  of  carbon 
and  hydrogen  must,  before  entering  the  combustion  tube,  be 
freed  from  water,  carbon  dioxide,  and  any  other  possible  con- 
stituents which  could  affect  the  weight  of  the  absorption  ap- 
paratus. The  arrangement  employed  to  effect  this  purification 
consists  of  (1)  a  U-tube  about  150  mm.  in  height,  which  is  filled 
with  soda-lime  or  with  small  pieces  of  potassium  hydroxide; 
(2)  another  U-tube  of  equal  size,  which  is  filled  with  calcium 
chloride  or  with  fragments  of  glass  (or  pumice  stone)  moistened 
with  concentrated  sulphuric  acid.  The  U-tubes  are  closed  with 
corks  carrying  small  glass  tubes  bent  to  a  right  angle,  and  the 
exposed  portions  of  the  cork  are  covered  with  sealing  wax.  In 


854  QUANTITATIVE  EXERCISES 

joining  the  two,  the  ends  of  the  small  glass  tubes  are  brought 
close  together  within  a  short  piece  of  rubber  tube,  which  is  then 
tied  and  painted  with  shellac.  When  not  in  use,  the  entrance 
and  exit  ends  of  the  apparatus  are  closed  with  rubber  tubes  and 
glass  plugs.  Whether  the  second  U-tube  is  to  contain  calcium 
chloride  or  sulphuric  acid  will,  of  course,  depend  upon  which  of 
these  reagents  is  to  be  employed  for  the  absorption  of  water. 

6.  AN  ABSORPTION  APPARATUS  FOR  WATER 

Whether  calcium  chloride  or  sulphuric  acid  is  used  as  the 
absorbent,  the  best  form  of  U-tube  for  the  collection  of  the  water 
is  that  which  is  opened  and  closed  by  means  of  ground-glass 
stoppers.  If  a  tube  of  this  kind  is  not  available,  the  "Mar- 
chand  "  tube  or  any  one  of  its  numerous  modifications  will  answer 
the  purpose.  The  tube  should  have  a  height  of  75  or  80  mm., 
and  an  internal  diameter  of  10  or  12  mm.  If  it  is  necessary  to 
close  it  with  stoppers,  corks  and  not  rubber  should  be  used, 
and  these  should  be  covered  with  sealing  wax. 

Calcium  chloride,  owing  to  the  never-failing  presence  of  the 
oxide,  must  be  exposed  for  a  long  time  to  an  atmosphere  of 
carbon  dioxide  before  employing  it  for  the  absorption  of  water. 
To  this  end,  the  tube  —  compactly  filled  with  small  pieces  of 
the  chloride  and  in  other  respects  ready  for  use  —  is  attached 
to  another  drying  tube,  which  is  in  turn  connected  with  a  car- 
bon dioxide  generator.  Before  weighing  the  tube  the  carbon 
dioxide  remaining  in  it  is  displaced  by  air.  As  previously 
stated,  the  oxide  in  calcium  chloride  cannot  be  wholly  con- 
verted into  carbonate  by  dry  carbon  dioxide.  For  this  reason 
the  contents  of  the  tube  must  be  frequently  renewed  and  never 
allowed  to  become  visibly  moist.  When  out  of  use  the  tube  is 
to  be  kept  in  a  desiccator  containing  calcium  chloride. 

Pumice  stone  usually  contains  chlorides  which  in  contact 
with  sulphuric  acid  yield  hydrochloric  acid.  Hence,  if  this 
material  is  to  be  employed  for  the  absorption  of  water,  the 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS      355 

pieces,  before  filling  the  tube  with  them,  must  be  heated  for  a 
considerable  time  with  the  concentrated  acid  in  order  to  insure 
the  decomposition  of  the  chlorides  and  the  removal  of  the  vola- 
tile hydrochloric  acid. 

7.  AN  ABSORPTION  APPARATUS  FOR  CARBON  DIOXIDE 

This  consists,  when  the  water  is  absorbed  by  calcium  chlo- 
ride, of  two  pieces,  —  a  Geissler  or  Liebig  apparatus,  pref- 
erably the  former,  for  liquid  potassium  hydroxide,  and  a  U- 
tube,  one  limb  of  which  is  filled  with  solid  potassium  hydroxide, 
and  the  other  with  calcium  chloride,  the  two  materials  being 
separated  by  a  thin  partition  of  ignited  asbestus  which  is  placed 
in  the  middle  of  the  horizontal  part  of  the  tube.  The  solution 
of  potassium  hydroxide  should  have  a  specific  gravity  of  about 
1.35,  and  the  bulbs  of  the  Geissler  apparatus  should  not  be 
more  than  one-half  full.  The  gas  which  passes  out  of  the 
combustion  tube  is  dried  by  the  calcium  chloride,  and  then 
gives  up  its  carbon  dioxide  in  the  Geissler  apparatus.  But  in 
passing  through  the  solution  of  potassium  hydroxide  it  acquires 
a  certain  quantity  of  water,  —  the  amount  depending,  of  course, 
on  the  temperature  and  concentration  of  the  solution.  It  is  to 
deprive  the  gas  of  this  water,  as  well  as  to  detain  any  carbon 
dioxide  which  may  have  escaped  absorption  in  the  Geissler 
apparatus  that  the  U-tube  filled  with  solid  absorbents  is  em- 
ployed. The  limb  of  the  tube  which  contains  the  alkali  is 
the  one  connected  with  the  Geissler  bulbs.  With  this  arrange- 
ment the  waste  gas  passes  last  over  calcium  chloride,  and  there- 
fore leaves  the  absorption  apparatus  for  carbon  dioxide  with  the 
same  tension  of  water  vapor  as  it  entered  it.  There  is,  of  course, 
a  slight  positive  error  due  to  the  change  in  the  volume  of  the 
gas  while  passing  through  the  alkaline  solution. 

If  the  water  is  determined  by  absorption  in  sulphuric  acid 
instead  of  calcium  chloride,  the  arrangement  of  the  apparatus 
for  the  collection  of  carbon  dioxide  must  be  somewhat  different. 


356  QUANTITATIVE  EXERCISES 

It  should  then  consist  of  (1)  a  Geissler  apparatus  containing  a 
solution  of  potassium  hydroxide,  (2)  a  tube  filled  with  solid 
potassium  hydroxide,  and  (3)  a  tube  filled  with  pumice  stone  or 
glass  beads  moistened  with  sulphuric  acid. 

8.  TUBES  FOR  PROTECTION  AGAINST  ATMOSPHERIC  WATER 
AND  CARBON  DIOXIDE 

The  contents  of  the  combustion  tube  and  of  the  various 
absorption  apparatuses  must  at  all  times  be  protected  against 
the  carbon  dioxide  and  the  water  of  the  air.  It  is  therefore 
necessary  to  have  at  hand  a  number  of  tubes  filled  with  absorb- 
ents for  these  compounds,  which  may  be  attached  wherever 
their  protection  is  required.  Tubes  filled  half  with  calcium 
chloride  and  half  with  soda-lime  will  suffice  for  the  protection 
of  a  weighed  absorption  apparatus  containing  calcium  chloride 
and  also  for  the  protection  of  the  combustion  tube,  if  the  water 
resulting  from  the  combustion  is  to  be  collected  in  calcium 
chloride.  But  if  an  absorption  apparatus  contains  sulphuric 
acid,  the  arrangement  employed  for  its  protection  against  the 
air  must  also  contain  this  acid.  Sulphuric  acid  should  likewise 
be  used  to  protect  the  contents  of  the  combustion  tube  when 
the  water  is  to  be  collected  in  that  acid. 


EXERCISE  XXXVIII 

DETERMINATION  OF  CARBON  AND  HYDROGEN 
I    THE  COMBUSTION  OF  A  SOLID  IN  THE  OPEN  TUBE 

(Applicable  to  compounds  not  containing  nitrogen) 

The  hard  glass  tube  which  is  used  in  this  exercise  should  be 
about  100  mm.  longer  than  the  furnace  and  should  have  an 
internal  diameter  of  15  or  16  mm.  The  rough  ends  are  to  be 
softened  in  the  flame  to  prevent  the  mutilation  of  the  stoppers, 
and  the  interior  of  the  tube  is  to  be  thoroughly  cleansed  and 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS   357 

dried.  Each  end  is  provided  with  a  tightly  fitting,  singly  per- 
forated rubber  stopper.  A  short  glass  tube  with  a  ground-glass 
stopcock  is  inserted  in  one  of  the  stoppers,  —  that  in  the  end 
through  which  oxygen  enters  the  combustion  tube.  The  other 
stopper,  which  receives  the  absorption  apparatus  for  water  when 
the  final  preparations  are  made,  should  fill  the  tube,  leaving  no 
space  between  it  and  the  glass  for  the  lodgment  of  water. 

The  manner  of  filling  the  tube  is  indicated  in  Fig.  54.  The 
space  a  is  filled  with  copper  oxide,  which  is  kept  in  place  by 
the  short  rolls  of  oxidized  copper  wire  gauze  b  and  bf.  The 
portion  of  this  space  nearest  b  and  b'  is  filled  with  the  coarser 
oxide,  and  the  remainder  with  the  finer.  The  space  c  is  occu- 
pied by  another  roll  of  oxidized  copper  wire  gauze  having  a 
length  of  100  or  120  mm.  The  space  d  is  left  vacant  for  the 
boat  which  is  to  contain  the  material  to  be  burned.  Behind  c?, 


a 


K- 320  rain-. M 

FIG.  54 

in  e,  is  still  another  roll  of  oxidized  copper  wire  gauze  70  or  75 
mm.  in  length,  and  to  this  is  attached  a  copper  wire  extend- 
ing nearly  to  the  end  of  the  tube.  The  rolls  6,  6',  c,  and 
e  are  made,  of  the  proper  dimensions,  from  closely  woven  wire 
gauze  and  oxidized  in  another  tube  by  conducting  over  them, 
while  hot,  a  current  of  dry  air  or  oxygen. 

Having  filled  the  tube  in  the  manner  indicated,  place  it  — 
without  the  boat  —  in  the  combustion  furnace,  taking  care  that 
the  trough  in  which  it  rests  is  straight  and  smoothly  lined  with 
asbestus  paper.  Attach  the  apparatus  for  the  purification  of 
oxygen  and  air  at/,  and  close  the  end  g  with  a  stopper  carrying 
an  un  weighed  calcium  chloride  tube.  Heat  the  portion  within  the 
furnace — slowly  and  as  uniformly  as  possible — to  dull  redness, 
and  aspirate  through  the  tube  a  slow  current  of  air.  When  it 
is  believed  that  the  contents  of  the  tube  have  been  sufficiently 


358  QUANTITATIVE  EXERCISES 

desiccated,  gradually  diminish  the  heat  without  interrupting  the 
current  of  air.  When  the  tube  is  cold  replace  the  calcium 
chloride  tube  at  g  with  the  weighed  absorption  apparatus  con- 
sisting of  (1)  the  calcium  chloride  tube,  (2)  the  Geissler  bulbs, 
(3)  the  U-tube  rilled  partly  with  solid  potassium  hydroxide  and 
partly  with  calcium  chloride,  and  (4)  an  unweighed  tube  con- 
taining calcium  chloride  and  soda-lime. 

Open  the  tube  at  f,  withdraw  the  roll  of  wire  gauze,  and 
quickly  insert  the  platinum  boat  containing  a  weighed  quantity 
of  the  purest  cane  sugar,  ranging  from  0.250  to  0.300  gram. 
Replace  the  roll  and  the  stopper,  leaving  the  combustion  tube 
connected  with  the  apparatus  for  the  purification  of  oxygen. 

Heat  the  roll  at  c  and  the  adjacent  half  of  the  copper  oxide 
to  a  dull  red,  and  then  light  the  burners  under  e.  Admit  a 
slow  current  of  oxygen  and  proceed  to  heat  the  remainder  of 
the  oxide,  advancing  gradually  toward  the  substance  in  the 
boat.  Regulate  the  temperature  in  the  vicinity  of  the  boat  so 
as  to  produce  a  slow  decomposition  of  the  sugar,  taking  care  in 
the  meantime  to  keep  the  whole  body  of  copper  oxide  at  a 
red  heat.  The  rapidity  of  the  decomposition  may  be  judged  by 
the  amount  of  carbon  dioxide  absorbed  in  the  Geissler  bulbs. 
During  the  early  stages  of  the  combustion,  while  the  sugar  is 
giving  off  volatile  matter,  only  a  very  slow  current  of  oxygen 
is  to  be  admitted  to  the  tube.  Later,  when  the  formation  of 
carbon  dioxide  has  nearly  ceased,  heat  the  tube  in  the  vicinity 
of  the  boat  to  redness  and  increase  somewhat  the  flow  of  oxy- 
gen. If,  as  usually  happens,  water  condenses  in  the  exit  end 
of  the  combustion  tube,  warm  the  part  where  water  is  seen  by 
inverting  over  the  projecting  end  of  the  tube  a  semicircular 
piece  of  sheet  iron  which  extends  for  a  short  distance  into  the 
furnace.  If  necessary,  in  order  to  effect  the  volatilization  of 
the  water,  the  iron  covering  may  in  turn  be  protected  by  a  piece 
of  asbestus  paper  to  prevent  loss  of  heat  by  radiation.  It  is 
hardly  safe  to  heat  the  tube  near  the  end  with  a  Bunsen  burner, 
owing  to  the  danger  of  burning  the  stopper. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS     359 

When  the  charred  mass  remaining  in  the  boat  has  been  com- 
pletely burned,  and  the  copper  reduced  from  the  oxide  during 
the  early  stages  of  the  combustion  has  been  reoxidized,  exchange 
the  current  of  oxygen  for  one  of  air  in  order  to  leave  the  absorp- 
tion apparatus  filled  with  the  latter  gas  rather  than  with  the 
former.  This  precaution  is  necessary  in  order  to  avoid  an  error 
in  weighing,  which  otherwise  would  result  from  the  fact  that 
oxygen  is  heavier  than  air. 

Disconnect  the  absorption  apparatus,  leaving  the  combustion 
tube  closed  with  an  unweighed  calcium  chloride  tube,  and  place 
the  various  pieces  to  be  weighed  —  protected  by  dry  rubber 
tubes  and  glass  plugs  —  in  the  balance  case.  After  half  an 
hour  remove  the  rubber  tubes  and  weigh. 

If  the  combustion  tube  is  slowly  and  uniformly  cooled,  it  is 
at  once  ready  for  a  second  determination. 

The  open  tube  may  also  be  used  for  the  determination  of 
carbon  and  hydrogen  in  compounds  containing  nitrogen.  In 
that  case,  however,  the  roll  of  wire  gauze  c  must  be  unoxidized 
and  must  also  be  protected  from  oxidation  during  the  combus- 
tion. To  this  end  the  roll  e  is  only  partially  oxidized  before 
beginning  the  combustion,  and  a  current  of  air  instead  of  oxygen 
is  employed.  No  oxygen  in  the  free  state  can  then  reach  c  until 
the  compound  has  been  burned  and  all  of  the  copper  behind 
it,  both  that  in  e  and  that  reduced  from  the  oxide,  has  been 
oxidized. 


II.    THE  COMBUSTION  OF  A  LIQUID  IN  THE  OPEN  TUBE 
(Applicable  to  compounds  not  containing  nitrogen) 

Any  liquid  organic  compound  which  is  free  from  water  and 
contains  neither  nitrogen  nor  sulphur  nor  a  halogen  may  be 
employed  in  this  experiment.  If  the  available  substance  is  an 
impure  compound  or  a  mixture  of  uncertain  composition,  e.g. 
coal  oil,  the  determination  should  be  duplicated. 


860  QUANTITATIVE  EXERCISES 

Soften  a  piece  of  small  glass  tubing  in  the  flame  of  a  Bunsen 
burner  and  draw  it  out  to  a  fine,  nearly  capillary  tube  about 
150  mm.  long.  Break  it  in  the  middle  and  separate  one  of 
the  smaller  tubes  from  the  larger  in  the  flame,  leaving  enough 
glass  for  a  small  bulb  upon  the  end  of  the  former.  Blow  at 
the  thicker  end  a  bulb  which  will  hold  about  half  a  cubic 
centimeter. 

Weigh  the  bulb.  Through  the  center  of  a  metallic  disk  about 
100  mm.  in  diameter  drill  a  small  hole  —  a  disk  of  tin  plate, 
galvanized  sheet  iron,  or  of  sheet  copper  will  answer  the  pur- 
pose —  and  place  the  disk  upon  a  shallow  beaker  or  other  vessel 
containing  a  quantity  of  the  liquid  to  be  burned.  Pass  the 
tube  of  the  bulb  through  the  hole  into  the  liquid  below,  and 
alternately  heat  and  cool  the  bulb  until  about  0.2  cc.  of  the 
liquid  have  ascended  into  it.  Remove  the  bulb,  cleanse  the 
portion  which  was  immersed,  close  the  open  end  in  the  flame, 
and  weigh  again. 

Reconnect  the  weighed  absorption  apparatus  and  the  com- 
bustion tube,  as  in  the  preceding  experiment,  and  heat  the  roll 
of  wire  gauze  c  and  the  adjacent  half  of  the  copper  oxide  to 
redness.  Make  a  file  mark  on  the  small  tube  near  the  bulb, 
and  place  the  bulb  in  a  boat  with  the  scratch  downward  and 
the  tube  extending  over  the  edge  of  the  boat.  Place  the  boat 
in  its  proper  position  within  the  combustion  tube,  and  break 
the  tube  in  the  boat  at  the  file  mark  by  pressing  upon  it  from 
above  with  a  rod  of  some  kind  which  is  sufficiently  long  for 
the  purpose.  Quickly  replace  the  roll  of  wire  gauze  e  and 
the  stopper,  and  then  conduct  the  combustion  precisely  as 
in.  the  preceding  experiment.  Somewhat  greater  caution  will 
be  required,  however,  owing  to  the  greater  volatility  of  the 
substance. 

If  a  liquid  containing  nitrogen  is  to  be  burned  in  the  open 
tube,  the  process  is  to  be  modified  in  the  same  manner  as  for 
a  solid  containing  this  element. 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS   361 

III.    COMBUSTION  IN  THE  CLOSED  TUBE 

It  is  recommended  that  some  solid  compound  containing 
nitrogen,  but  free  from  sulphur  or  a  halogen,  be  selected  for 
this  experiment. 

Figure  55  represents  the  "  bayonet "  tube,  which  is  the  form 
usually  employed.  Its  length  is  to  be  so  regulated  that,  when 
finished,  each  end  extends  from  50  to  75  mm.  beyond  the 
furnace. 

After  cleansing  and  drying  the  tube,  place  it  in  the  furnace 
and  attach  to  each  end  a  calcium  chloride  drying  apparatus. 
Heat  the  tube  to  a  uniform  dull  red  and  aspirate  through  it 
a  current  of  air.  This  treatment  of  the  glass  should  be  con- 
tinued for  an  hour  or  more  in  order  to  insure  its  complete  dehy- 
dration. Discontinue  the  current  of  air  and  cool  the  tube  very 


I  c  | 


FIG.  55 

slowly  and  uniformly  in  order  to  secure  a  good  annealing  of 
the  glass.  Without  removing  the  drying  apparatus  fuse  off 
the  "bayonet"  end  of  the  tube  at  a  point  near,  but  not  too 
near,  the  rubber  which  connects  it  with  the  calcium  chloride 
tube. 

The  manner  of  filling  the  tube  will  be  indicated  by  reference 
to  the  figure. 

a  is  a  short  plug  of  ignited  asbestiis  which  prevents  the  copper 
oxide  from  entering  the  bayonet.  To  prepare  the  asbestus  for 
this  use,  it  is  heated  in  a  platinum  dish  over  the  blast  lamp  and 
placed,  while  still  warm,  in  a  desiccator. 

b  is  a  layer  of  coarse  copper  oxide  about  100  mm.  in  length. 

c  is  a  layer  of  fine  copper  oxide  about  50  mm.  in  length. 

d,  a  space  100  mm.  in  length,  or  longer  if  the  substance  is 
very  volatile  or  the  quantity  of  it  is  large,  is  occupied  by  the 
substance  to  be  burned  and  by  more  of  the  fine  oxide.  In  filling 


362  QUANTITATIVE   EXERCISES 

the  tube  the  layer  c  is  introduced,  then  the  boat  containing 
the  compound,  and  finally  as  much  of  the  finer  oxide  as  may  be 
thought  necessary.  By  many  b  and  c  are  separated  by  a  short 
plug  of  asbestus  to  prevent  the  finer  oxide  from  sifting  into  the 
coarser.  This  precaution  is  hardly  necessary,  however,  unless 
the  two  grades  of  oxide  differ  widely  in  respect  to  fineness. 

e  is  a  layer  of  coarse  copper  oxide  about  200  mm.  long. 

/  is  a  roll  of  bright  copper  wire  gauze  (or  a  layer  of  metallic 
copper  reduced  from  the  oxide)  100  mm.  or  more  in  length. 
The  purpose  of  the  metallic  copper  is  to  reduce  oxides  of  nitro- 
gen; it  may,  of  course,  be  omitted  and  the  tube  made  cor- 
respondingly shorter  when  the  substance  to  be  burned  contains 
no  nitrogen. 

g  is  a  layer  of  coarse  copper  oxide,  or  a  roll  of  oxidized  wire 
gauze,  about  50  mm.  in  length,  which  serves  to  burn  any  car- 
bon monoxide  formed  from  the  dioxide  while  passing  over  the 
metallic  copper.  Such  reduction  of  the  dioxide  is  improbable 
unless  the  copper  contains  iron,  and  the  precaution  of  placing 
the  oxide  before  the  metal  is  therefore  often  dispensed  with. 

The  tube  should  be  filled  with  the  least  possible  exposure 
of  the  contents  to  the  moisture  of  the  air.  To  this  end  it  is 
well  to  have  all  necessary  materials  at  hand  before  beginning, 
to  open  the  tube  and  the  flasks  containing  the  copper  oxide 
only  when  necessary,  and  to  close  them  as  quickly  as  possible. 
These  precautions  are  made  necessary  by  the  highly  hygro- 
scopic character  of  copper  oxide.  It  is  the  common  experi- 
ence, however,  that  the  closed-tube  method  gives  somewhat  too 
high  percentages  of  hydrogen  in  spite  of  all  possible  care  in 
manipulation. 

Place  the  filled  tube  in  the  furnace,  make  a  file  mark  across 
it  close  to  the  small  end  of  the  bayonet,  and  attach  the  weighed 
absorption  apparatus.  Draw  a  little  air  out  of  the  apparatus 
by  suction  and  then  allow  it  to  reenter  slowly.  The  solution 
of  potassium  hydroxide  in  the  Geissler  bulbs  will  rise  into  the 
upper  bulb  of  the  part  nearest  the  combustion  tube,  and  if  the 


CAKIJOX    AND  HYDROGEN   IN  ORGANIC  COMPOUNDS      363 

joints  are  all  perfect,  the  level  of  the  liquid  will  thereafter 
remain  unchanged.  If  the  liquid  gradually  descends,  the  leaky 
connection  must  be  sought  out  and  made  tight. 

Heat  #,  the  right-hand  half  of  e,  and  a  little  of  each  end  of 
/to  redness.  Afterwards  heat  the  portion  of  b  nearest  a  and 
the  remainder  of  /  also  to  redness.  Having  thus  provided  for 
the  combustion  of  any  moderate  quantity  of  volatile  matter 
and  the  reduction  of  oxides  of  nitrogen,  gradually  heat  the 
remaining  copper  oxide,  advancing  from  both  directions  towards 
the  substance,  and  so  regulating  the  temperature  in  its  vicinity 
as  to  produce  a  slow  combustion. 

Insert  the  apparatus  for  the  purification  of  oxygen  and  air 
between  the  bayonet  and  the  gasometer,  and  admit  enough 
oxygen  to  produce  a  slight  overpressure.  When  the  solution 
of  potassium  hydroxide  rises  in  the  Geissler  bulbs  in  the  direc- 
tion of  the  combustion  tube,  break  off  the  end  of  the  bayonet, 
admitting  the  oxygen,  and  heat  the  copper  oxide  in  the  vicinity 
of  the  boat  to  redness.  As  soon  as  the  copper  reduced  from 
the  oxide  appears  to  be  reoxidized,  and  before  the  roll  of  wire 
gauze  is  attacked,  exchange  the  current  of  oxygen  for  one  of 
air  and  allow /to  cool.  The  remaining  steps  are  the  same  as 
in  the  preceding  experiments. 

The  procedure  is  not  materially  changed  for  the  combustion 
of  a  liquid.  The  substance  is  introduced  into  the  tube  in  a 
bulb,  as  in  experiment  II,  and  the  stem  is  broken  off  in  the  same 
manner  at  a  file  mark  made  near  the  bulb.  The  heating  of  the 
tube  requires,  of  course,  somewhat  greater  care  than  when  a 
solid  is  burned. 


IV.    THE  COMBUSTION  OF  A  COMPOUND  CONTAINING  SULPHUR 

The  compound  selected  for  this  experiment  may  also  contain 
nitrogen  and  it  may  be  burned  in  the  open  or  in  the  closed 
tube.  The  necessary  modifications  of  previous  procedure  are 
given  on  page  364. 


364  QUANTITATIVE  EXERCISES 

a.    Combustion  in  the  Open  Tube 

The  tube  is  filled  as  directed  under  I,  Exercise  XXVIII, 
except  in  the  following  particulars :  (1)  the  space  a  is  filled 
with  lead  chromate  prepared  by  de  Roode's  method,  instead  of 
with  copper  oxide ;  (2)  if  the  compound  to  be  burned  contains 
nitrogen,  the  roll  c  is  to  be  substituted  by  one  of  unoxidized 
copper  wire  gauze,  and  the  roll  e  is  to  be  oxidized  only  partially. 

The  burning  of  the  compound  is  conducted  as  directed  under 
I,  except  in  the  case  of  nitrogenous  substances,  which  are  better 
burned  in  a  current  of  air  than  in  one  of  oxygen. 

b.    Combustion  in  the  Closed  Tube 

The  tube  is  filled  as  directed  under  III,  except  that  lead 
chromate  is  used  throughout  in  place  of  copper  oxide,  and  the 
process  of  burning  is  the  same  as  when  the  closed-tube  method 
is  employed  for  compounds  which  are  free  from  sulphur. 

V.   THE  COMBUSTION  OF  A  COMPOUND  CONTAINING  A  HALOGEN 

When  an  organic  compound  containing  a  halogen  is  burned 
with  copper  oxide,  a  cuprous  salt  is  formed  which  at  a  red  heat 
volatilizes  and  is  decomposed  by  free  oxygen  into  copper  oxide 
and  the  halogen. 

There  are  various  ways  of  obviating  this  difficulty,  of  which 
the  following  are  the  best : 

1.  The  substance  is  burned  with  lead  chromate,  in  which 
case  a  lead  salt  of  the  halogen  is  formed.     The   open  or  the 
closed  tube  may  be  used.     It  is  well,  however,  in  either  case 
to  heat  the  chrcmate  near  the  exit  end  of  the  tube  only  moder- 
ately.   If  the  substance  contains  nitrogen  also,  the  roll  of  copper 
wire  gauze  or  a  layer  of  the  metal  reduced  from  the  oxide  must 
be  placed  in  the  tube  in  the  usual  position. 

2.  The   substance  is  burned  in  the  ordinary  manner,  with 
copper  oxide  either  in  the  open  or  in  the  closed  tube,  but  a  roll 


CARBON  AND  HYDROGEN  IN  ORGANIC  COMPOUNDS   365 

of  bright  silver  foil  150  mm.  in  length  is  placed  in  the  exit 
end  of  the  tube  and  moderately  heated  during  the  combustion. 
If  the  substance  contains  nitrogen,  a  roll  of  copper  wire  gauze 
must  also  be  used. 

When  organic  compounds  containing  metals  of  the  alkalies  or 
of  the  alkaline  earths  are  burned,  stable  carbonates  are  formed; 
it  is  therefore  necessary  in  such  cases  to  add  to  the  oxidizing 
material  some  substance  which  is  capable  of  decomposing  carbon- 
ates. These  compounds  are  usually  burned  with  a  mixture  of 
lead  chromate  (10  parts)  and  of  potassium  dichromate  (1  part). 
The  results  are  then  normal ;  for,  at  a  red  heat, 


K2Cr207+  K2C03  =  2  K2CrO4 

K2Cr2O7  +  BaCOg  =  K2CrO4  +  BaCrO4  +  CO2,  etc. 

The  methods  employed  in  the  preceding  exercise  provide 
only  for  the  determination  of  the  carbon  and  hydrogen  of 
organic  compounds.  There  are  other  methods,  not  so  much 
used,  which  accomplish  a  simultaneous  determination  of  carbon, 
hydrogen,  and  nitrogen,  and  still  others  which  enable  one  to 
determine  the  oxygen  in  organic  compounds.  Respecting  these, 
as  well  as  some  other  methods  which  differ  essentially  from 
those  already  employed,  the  student  is  recommended  to  consult 
the  following  sources : 

Fresenius,  Quant.  Analyse,  2,  97-110. 
Wheeler,  Jour.  prak.  Chem.,  96,  239. 

Zeitsch.  anal.  Chem.,  5,  217. 
Schulze,  Zeitsch.  anal.  Chem.,  5,  269. 
Frerichs,  Ber.  d.  d.  chem.  Ges.,  10,  26. 
Kopfer,  Zeitsch.  anal.  Chem.,  17,  1. 
Hempel,  Zeitsch.  anal.  Chem.,  17,  409. 


CHAPTER  XVI 

THE  DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL.     THE 
INCINERATION  OF  ORGANIC  SUBSTANCES.     THE  DETER- 
MINATION OF  ORGANIC  MATTER  IN  WATER 


EXERCISE  XXXIX 

DETERMINATION  OF  TOTAL  CARBON  IN  IRON  AND  STEEL 
I.    BY  DIRECT  COMBUSTION  IN  A  CURRENT  OF  OXYGEN 

There  are  required  for  this  experiment.: 

1.  About  3  grams  of  steel  filings  or  fine  turnings  free  from 
grease  and  other  impurities. 

2.  A  glazed  porcelain  tube  having  an  internal  diameter  of  16 
or  18  mm.     The  required  length  of  the  tube  depends  upon  the 
length  of  the  furnace  in  which  it  is  to  be  used.     A  short  fur- 
nace with  12  or  15  burners  will  suffice,  and  it  is  necessary  only 
that  the  tube  shall  be  from  150  to  200  mm.  longer  than  the 
furnace.     A  short  roll  of  oxidized  copper  wire  gauze  is  placed 
in  the  tube  near  the  middle ;  and  one  of  the  two  spaces  into 
which  the  tube  is  thus  divided  is  completely  filled  with  granu- 
lar copper  oxide  to  a  point  which,  during  the  combustion,  will 
lie  near  the  end  of  the  furnace.     Another  roll  of  oxidized  wire 
gauze  is  then  inserted  to  keep  the  oxide  in  place.     There  is 
also  prepared  a  third  roll  of  oxidized  wire  gauze  100  mm.  in 
length,  to  which  is  attached  a  copper  wire  with  a  loop  on  the 
end.     This,  as  is  customary  in  all  similar  combustions,  is  placed 

366 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL   367 

in  the  rear  end  of  the  tube  after  the  insertion  of  the  boat 
containing  the  material  to  be  burned.  Each  end  of  the  porcelain 
tube  is  provided  with  a  rubber  stopper  through  which  is  passed 
a  small  glass  tube.  For  the  better  protection  of  the  stoppers 
against  radiated  heat,  the  glass  tubes  are  made  to  project  a  few 
millimeters  into  the  porcelain  tube,  and  over  the  projecting  ends 
are  placed  perforated  disks  cut  from  thin  asbestus  board.  Any 
organic  matter  which  these  disks  may  contain  is  destroyed  by 
heating  them  for  a  time  to  redness  in  a  platinum  dish. 

3.  A  supply  of  oxygen  in  a  gasometer,  and  the  usual  appa- 
ratus for  the  purification  of  that  gas. 

4.  An  apparatus  to  be  attached  to  the  exit  end  of  the  porcelain 
tube  for  the  absorption  of  water  and  sulphur  dioxide.     This  con- 
sists of  a  U-shaped  tube,  the  vertical  limbs  of  which  are  filled  with 
calcium  chloride,  and  the  horizontal  portion  with  lead  superoxide, 
the  two  reagents  being  separated  by  asbestus  or  cotton  wool. 

5.  An  absorption  apparatus  for  carbon  dioxide  which,  as  in 
the  combustion  of  organic  compounds,  consists  of  (1)  a  Geissler 
apparatus  containing  a  solution  of  potassium  hydroxide  (sp.  gr. 
1.35),    and    (2)  a    U-tube    filled    partly   with    solid    potassium 
hydroxide  and  partly  with  calcium  chloride,  the  two  absorbents 
being  separated  by  asbestus. 

6.  A  tube  filled  partly  with  calcium  chloride  and  partly  with 
soda-lime  to  protect  5  from  the  carbon  dioxide  and  the  moisture 
of  the  air. 

Heat  the  porcelain  tube,  with  4  in  place,  to  redness  —  raising 
the  temperature  very  gradually  —  and  pass  dry  oxygen  through 
it.  Cool  it  slowly,  and  when  cold  attach  5.  Test  the  appa- 
ratus for  leaks  and  then  heat  to  redness  the  portion  of  the  tube 
which  contains  the  copper  oxide.  Spread  the  weighed  steel 
filings  evenly  over  the  bottom  of  a  capacious  porcelain  boat. 
Insert  the  boat  and  the  roll  of  oxidized  wire  gauze,  and  conduct 
the  combustion  as  directed  in  Exercise  XXXVIII  under  VI.  The 
tube  should  be  maintained  at  a  red  heat  for  nearly  an  hour 
before  exchanging  the  current  of  oxygen  for  one  of  air. 


368  QUANTITATIVE  EXERCISES 

This  method  is  applicable  to  the  determination  of  total  carbon 
in  all  forms  of  iron.  It  is  necessary,  however,  that  the  material 
be  quite  finely  divided,  since  otherwise  a  portion  of  the  carbon 
may  escape  oxidation. 

The  total  carbon  in  iron,  like  carbon  in  organic  compounds, 
may  also  be  determined  by  combustion  in  the  closed  glass  tube. 
The  essential  features  of  the  procedure  are  given  below. 

1.  The  material  in  a  finely  divided  condition  is  mixed  in  a 
porcelain  mortar  with  15  parts  by  weight  of  powdered  lead 
chromate  and  1.5  parts  of  potassium  chlorate  or  the  same  weight 
of  potassium  bichromate.     A  little  ignited  asbestus  is  placed  in 
the  bayonet  end  of  the  glass  tube,  and  upon  this  a  layer  of  pow- 
dered lead  chromate,  25  mm.  in  length.    The  mixture  containing 
the  iron  is  then  introduced  through  a  funnel,  and  thereafter 
several  small  portions  of  the  lead  chromate  which  have  been 
previously  used  to  rinse  the  mortar.     The  filled  tube  is  held  in 
a  horizontal  position  and  gently  tapped  to  produce  a  channel  for 
the  passage  of  gas.     If  potassium  chlorate  has  been  mixed  with 
the  iron,  the  tube  which  is  attached  to  the  exit  end  of  the  com- 
bustion tube  for  the  absorption  of  water  must  contain  a  quantity 
of  anhydrous  copper  sulphate  as  well  as  calcium  chloride.     The 
combustion  is  carried  out  in  the  usual  manner. 

2.  The  filings  or  powdered  material  are  mixed  in  a  mortar 
with  20  or  more  parts  by  weight  of  fine  copper  oxide.     The 
tube  is  filled  in  the  same  manner  as  in  1  except  that  copper  oxide 
is  used  throughout  in  the  place  of  lead  chromate.     For  the 
absorption  of  water  and  sulphur  dioxide,  the  apparatus  described 
under  4  in  the  preceding  exercise  is  employed.     The  combustion 
is  conducted  in  a  current  of  oxygen. 

II.    BY  OXIDATION  WITH  CHROMIC  ACID 

There  are  required  : 

1.  A  flask  having  a  capacity  of  300  cc.  This  is  provided 
with  a  doubly  perforated  rubber  stopper.  Through  one  of  the 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      369 

holes  is  passed  a  short  glass  tube  bent  to  a  right  angle,  and 
through  the  other  a  separating  funnel  with  a  glass  stopcock. 
The  funnel  should  have  a  capacity  of  not  less  than  100  cc.,  and 
the  orifice  at  the  delivering  end  should  be  small.  The  right- 
angled  tube  serves  for  the  exit  of  the  gas,  and  the  separating 
funnel  for  the  introduction  of  the  reagents. 

2.  An  apparatus  for  the  purification  of  air.     This  consists  of 
a  U-tube  filled  with  soda-lime  or  with  pieces  of  solid  potassium 
hydroxide.     It  is  connected  with  the  upper  end  of  the  separating 
funnel  by  means  of  a  tube  bent  to  two  right  angles. 

3.  A  series  of  three  connected  U-tubes  in  which  the  carbon 
dioxide  is  dried  before  entering  the  weighed  absorption  appa- 
ratus.    The  first,  that  nearest  the  flask  1,  is  empty,  while  the 
second  and  third  are  filled  with  calcium  chloride. 

4.  The  usual  apparatus  for  the  absorption  of  carbon  dioxide, 
together  with  the  tube  for  the  protection  of  its  contents  from 
the  carbon  dioxide  and  the  moisture  of  the  air. 

Weigh  about  one  gram  of  pig-iron  drillings  into  the  flask. 
Connect  the  different  parts  of  the  apparatus  and  test  the  whole 
for  leaks.  Introduce  into  the  flask  through  the  funnel  from  10 
to  15  cc.  of  a  saturated  solution  of  chromic  acid,  and  then  very 
cautiously  100  cc.  of  concentrated  sulphuric  acid  which  has  been 
heated  to  the  boiling  point  with  a  little  chromic  acid,  and  after- 
wards, when  cold,  allowed  to  stand  over  crystals  of  chromic  acid 
until  saturated.  Introduce  also  in  the  same  manner  50  cc.  of 
dilute  sulphuric  acid  having  a  specific  gravity  of  1.10.  Heat 
the  flask  gradually  nearly  to  the  boiling  point  of  the  solution. 
When  the  evolution  of  gas  ceases,  conduct  two  or  three  liters  of 
air  through  the  apparatus,  slowly  lowering  the  temperature  of 
the  flask  meanwhile. 

The  foregoing  processes  1  and  2  provide  for  the  determina- 
tion of  total  carbon  in  iron  and  steel  without  previous  removal 
of  the  iron  in  the  sample.  There  are  also  various  methods  by 
which  the  iron  may  be  separated  from  the  carbon  in  such  a 
manner  as  to  leave  the  whole  of  the  latter  in  the  form  of  an 


370  *  QUANTITATIVE  EXERCISES 

insoluble  residue.  The  carbon  in  such  residues  is  not  pure,  and 
it  is  therefore  necessary  to  burn  them  and  to  collect  the  result- 
ing carbon  dioxide  in  a  weighed  absorption  apparatus.  The 
following  are  some  of  the  methods  by  which  the  separation  of 
iron  and  carbon  is  effected. 

1.  The  material  is  treated  with  a  saturated  solution  of  the 
double  chloride  of  copper  and  ammonium,  and  a  quantity  of 
concentrated  hydrochloric  acid,  and  afterwards  stirred  until  the 
precipitated  copper  is  redissolved.     The  residue  of  carbon  is 
collected  upon  an  asbestus  filter,  —  preferably  upon  one  which 
has  been  made  in  a  platinum  boat  with  a  perforated  bottom,  — 
washed,  dried,  and  burned.     Instead  of  the  ammonium  salt  the 
double  chloride  of  copper  and  potassium  may  be  employed  to 
dissolve  the  iron. 

2.  The  sample,  in  a  single  piece  if  practicable,  is  placed  in  a 
basket  of  closely  woven  platinum  wire,  which  is  suspended  in 
a  beaker  and  connected  with  the  positive  pole  of  a  single  Bun- 
sen  or  Grove  element.     In  the  same  beaker,  but  connected  with 
the  other  pole  of  the  battery,  is  a  piece  of  platinum  foil.     The 
beaker  is  filled  to  the  requisite  depth  with  a  solution  of  1  part 
by  volume    of   concentrated  hydrochloric   acid  in  4  parts   of 
water,  and  the  current  is  so  controlled  by  the  introduction  of 
resistance    that  no    gas    escapes  from   the  iron.     The  residue 
of  carbon,  which  usually  retains  the  form  of  the  sample,  is 
transferred   to    an    asbestus    filter,  washed,   dried,   and   finally 
burned. 

3.  The  material,  in  a  porcelain  boat,  is  placed  in  a  glass  tube 
and  heated  in  a  current  of  chlorine  until  all  of  the  iron  has  been 
volatilized  in  the  form  of  ferric  chloride.     The  residue  contains 
the  carbon,  also  any  slag  and  manganese  which  may  have  been 
in  the  sample.     It  is  transferred  to  an  asbestus  filter,  washed, 
to  free  it  from  nonvolatile  chlorides,  dried,  and  burned. 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      371 

| 

III.   BY  OXIDATION  OF  THE  CARBON  AFTER  REMOVAL 
OF  THE  IRON 

Treat  about  one  gram  of  pig-iron  drillings  in  a  beaker  with 
100  cc.  of  a  saturated  solution  of  copper-ammonium  chloride 
and  7.5  cc.  of  strong  hydrochloric  acid.  Stir  the  solution  vig- 
orously for  some  minutes  and  then  place  the  beaker  in  a  warmed 
bath  but  not  where  its  temperature  can  rise  above  60°.  Con- 
tinue to  stir  at  frequent  intervals  until  the  precipitated  copper 
is  dissolved.  Allow  the  undissolved  material  to  subside  and 
then  filter  through  ignited  asbestus. 

A  suitable  filter  may  be  made  in  the  following  manner :  A 
perforated  platinum  or  porcelain  disk  is  placed  in  the  bottom 
of  a  cylindrical  glass  funnel  like  that  used  with  the  Gooch 
crucible,  and  upon  it  is  made,  in  the  usual  manner,  a  filter 
of  asbestus  which  has  been  previously  ignited. 

Wash  the  precipitate  and  dry  it  at  100°.  Transfer  it,  together 
with  the  filter,  to  a  platinum  or  porcelain  boat.  Wipe  out  the 
funnel  with  ignited  asbestus,  and  add  the  material  so  employed 
to  the  boat.  Burn  the  carbon  as  directed  under  I,  having 
previously  placed  a  roll  of  silver  foil  in  the  exit  end  of  the 
porcelain  combustion  tube. 

Repeat  the  determination,  oxidizing  the  carbon  with  chromic 
acid.  The  apparatus  required  for  this  purpose  is  the  same  as 
that  described  under  II  except  as  regards  the  arrangement  for 
the  purification  of  the  carbon  dioxide.  This  consists  of  (1)  a 
Peligot  tube  containing  a  solution  of  silver  sulphate  in  concen- 
trated sulphuric  acid ;  (2)  an  empty  U-tube ;  (3)  a  U-tube  filled 
with  pumice  stone  which  has  been  saturated  with  a  concentrated 
solution  of  copper  sulphate  and  subsequently  heated  to  dehy- 
drate the  salt ;  (4)  a  tube  filled  with  granular  calcium  chloride. 
The  apparatus  for  the  absorption  of  carbon  dioxide  is  the  same 
as  that  usually  employed. 

Transfer  to  the  flask  the  asbestus  filter  on  which  the  material 
to  be  burned  has  been  collected  and  washed,  and  also  the  asbestus 


372  QUANTITATIVE  EXERCISES 

used  in  cleaning  the  funnel.  Add  through  the  separating 
funnel  10  cc.  of  a  saturated  water  solution  of  chromic  acid  and 
then  100  cc.  of  concentrated  sulphuric  acid  which  has  been 
heated  nearly  to  the  boiling  point  with  a  little  chromic  acid. 
Pass  a  slow  current  of  air  through  the  apparatus,  and  very 
gradually  raise  the  temperature  of  the  liquid  in  the  flask  nearly 
to  the  boiling  point.  When  the  oxidation  is  complete  slowly 
cool  the  flask,  continuing  for  a  time  the  current  of  air. 


EXERCISE  XL 
DETERMINATION  OF  GRAPHITIC  CARBON  IN  IRON 

Treat  1  gram  of  pig-iron  drillings  with  15  cc.  of  nitric  acid 
of  1.2  specific  gravity.  Collect  the  undissolved  material  upon 
an  asbestus  filter  and  wash  it  thoroughly  with  hot  water.  Treat 
the  residue  on  the  filter  with  a  hot  solution  of  potassium  hydrox- 
ide of  1.1  specific  gravity.  Wash  thoroughly  with  water,  after- 
wards with  dilute  hydrochloric  acid,  and  finally  with  water. 
After  drying,  burn  the  carbon  by  either  of  the  methods  pre- 
viously used. 

Repeat  the  determination,  proceeding  in  the  following  man- 
ner: Treat  1  gram  of  the  iron  with  an  excess  of  hydrochloric 
acid  of  1.1  specific  gravity.  When  the  solution  of  the  iron  is 
complete,  boil  for  a  few  minutes  and  then  allow  the  graphite  to 
subside.  Decant  through  an  asbestus  filter.  Wash  by  decan- 
tation  several  times,  using  hot  water.  Treat  the  residue  with 
30  cc.  of  a  solution  of  potassium  hydroxide  of  1.1  specific  grav- 
ity, and  when  effervescence  ceases,  heat  the  solution  to  the  boil- 
ing point.  Collect  the  graphite  upon  the  asbestus  filter  and 
wash  first  with  water  and  finally  with  alcohol  and  ether.  Burn 
the  carbon  in  the  same  manner  as  before. 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      373 

EXERCISE  XLI 
DETERMINATION  OF  COMBINED  CARBON  IN  STEEL 

COLORIMETRTC    METHOD    OF    EGGERTZ 

This  method  depends  upon  the  fact  that  when  samples  of 
similar  varieties  of  steel  or  of  iron  are  dissolved  in  nitric  acid 
of  1.2  specific  gravity,  the  depth  of  the  brown  color  which  is 
developed  in  the  several  solutions  is  proportional  to  the  quan- 
tity of  the  chemically  combined  carbon  in  the  samples.  It  is 
therefore  practicable  to  determine  the  combined  carbon  in  any 
specimen  of  steel  by  comparing  the  color  developed  on  dissolv- 
ing a  known  weight  of  it  in  nitric  acid  with  the  color  in  a  stand- 
ard solution  which  has  been  made  by  dissolving  a  weighed 
quantity  of  a  similar  steel  whose  percentage  of  carbon  is  known. 
If  the  darker  of  the  two  solutions  —  the  solution  of  the  stand- 
ard and  that  of  the  sample  to  be  analyzed  —  is  diluted  until  both 
exhibit  the  same  shade  of  color,  equal  volumes  of  them  will 
contain  the  same  quantity  of  carbon,  and  the  total  quantities  of 
carbon  in  them  will  be  related  as  the  volumes  of  the  solutions. 

The  percentage  of  carbon  in  the  steel  which  is  to  serve  as  the 
standard  must  be  determined  by  combustion  by  one  of  the 
methods  already  given.  For  this  particular  experiment  it  is 
recommended  that  the  steel  employed  in  Exercise  XXXIX,  I, 
be  selected  for  the  standard,  and  that  two  or  three  other  steels 
of  the  same  general  character  and  the  same  history  as  regards 
previous  treatment  be  chosen  as  the  material  for  analysis. 
Owing  to  the  well-established  fact  that  the  condition  of  chem- 
ically combined  carbon  in  steel  may  be  decidedly  modified  by 
mechanical  means,  such  as  rolling,  hammering,  tempering,  etc., 
it  is  quite  essential  that  only  those  steels  which  have  received 
the  same  treatment  shall  be  compared  with  each  other. 

Arrange  in  a  rack  a  series  of  clean,  dry,  and  numbered  test 
tubes  having  a  length  of  150  mm.  and  a  diameter  of  15  or  16  mm. 
Weigh  into  one  of  the  tubes  about  0.2  gram  of  the  standard 


374  QUANTITATIVE  EXERCISES 

steel  and  into  the  others  nearly  equal  quantities  of  the  samples 
to  be  tested.  Place  the  rack  in  a  bath  of  cold  water  and  add  to 
each  test  tube  from  a  graduated  pipette  the  required  amount  of 
nitric  acid  of  1.2  specific  gravity,  —  i.e.  for  steels  containing  less 
than  0.3  per  cent  of  carbon,  3  cc. ;  from  0.3  to  0.5  per  cent, 
4  cc. ;  from  0.5  to  0.8  per  cent,  5  cc. ;  from  0.8  to  1.0  per  cent, 
6  cc. ;  over  1.0  per  cent,  7  cc.  If  the  percentage  of  carbon  in 
the  samples  is  not  even  approximately  known,  begin  by  adding 
3  cc.  of  the  acid  to  each,  and  afterwards  increase  the  quantity, 
1  cc.  at  a  time,  as  the  depth  of  color  and  the  presence  of  undis- 
solved  carbonaceous  matter  in  the  solution  indicate  the  need  of 
more  acid.  The  nitric  acid  must  be  wholly  free  from  chlorine. 
Place  a  small  funnel  in  the  mouth  of  each  test  tube.  Heat  the 
water  in  the  bath  to  the  boiling  point  and  maintain  it  at  that 
temperature  —  shaking  the  tube  from  time  to  time  —  until  all 
of  the  carbonaceous  matter  is  dissolved.  Remove  each  tube  to 
a  bath  of  cold  water  as  soon  as  the  solution  in  it  becomes  clear, 
taking  care  to  exclude  any  strong  light  and  especially  direct 
sunlight. 

Transfer  the  various  solutions,  together  with  the  water  used 
in  rinsing  the  test  tubes,  to  Eggertz  graduated  tubes.  These 
are  clear  and  colorless  glass  tubes  of  uniform  caliber,  which 
have  been  graduated  from  the  bottom  upwards  for  a  capacity 
of  thirty  or  more  cubic  centimeters. 

Dilute  the  solution  of  lightest  color  to  about  twice  the 
volume  of  the  acid  which  was  used  in  making  it,  and  then 
dilute  all  the  other  solutions  to  the  shade  presented  by  the 
first  after  its  dilution.  In  comparing  shades  the  solutions  are, 
of  course,  viewed  transversely  and  not  in  a  vertical  direction, 
and  the  comparison  is  usually  facilitated  by  placing  the  tubes 
containing  the  solutions  to  be  compared  in  a  rack  with  a  white 
background. 

If  the  quantity  of  carbon  known  to  be  in  the  weighed  sample 
of  the  standard  steel  is  divided  by  the  volume  of  the  stand- 
ard solution  in  cubic  centimeters,  the  quotient  will  be  the 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      375 

quantity  of  carbon  represented  by  one  cubic  centimeter  of  that 
or  of  any  other  solution  of  the  series.  Hence,  to  find  the  weight 
of  carbon  in  any  one  of  the  samples  under  examination,  it 
is  necessary  only  to  multiply  the  volume  of  its  solution  by  this 
quotient. 

Permanent  color  standards  are  also  employed  in  the  determi- 
nation of  carbon  in  steel.  These  are  made  by  dissolving  in 
water  certain  definite  quantities  of  hydrochloric  acid  and  of  the 
chlorides  of  iron,  copper,  and  cobalt. 


EXERCISE  XLII 
DETERMINATION  OF  THE  FUEL  VALUE  OF  COAL 

.  The  exact  analysis  of  coals  and  the  precise  estimation  of  their 
value  as  fuel  are  problems  of  considerable  complexity,  and  they 
cannot  therefore  be  considered  in  this  place.  There  are,  how- 
ever, certain  expeditious  processes  which,  though  no  high  degree 
of  exactness  can  be  claimed  for  them,  nevertheless  enable  one 
to  ascertain  the  general  character  of  coals  and  to  determine 
approximately  their  relative  value  for  heating  purposes.  The 
method  here  presented  is  of  this  kind.  It  provides  for  the 
determination  of  (1)  the  water,  (2)  the  volatile  combustible  mat- 
ter, (3)  the  fixed  or  nonvolatile  carbon,  and  (4)  the  sum  of  the 
inorganic  or  ash-forming  constituents. 

Great  care  must  be  exercised  in  securing  a  sample  which 
shall  fairly  represent  the  average  composition  of  the  whole  sup- 
ply from  which  it  is  drawn.  To  this  end,  a  shovelful  of  coal  is 
taken  from  many  parts  of  the  stock.  The  small  portions  thus 
withdrawn  are  thoroughly  mixed  in  one  heap,  which  is  after- 
wards divided  into  two  equal  parts.  One  of  the  halves  is  again 
mixed  and  divided  as  before.  The  process  of  mixing  and  halv- 
ing is  continued  until  the  material  has  been  reduced  to  a  man- 
ageable quantity,  when  the  larger  pieces  .are  broken  into  smaller 
ones.  The  process  of  mixing  and  halving  is  then  resumed; 


376  QUANTITATIVE  EXERCISES 

but  after  each  partition  the  portion  selected  for  further  manip- 
ulation is  reduced  to  a  still  finer  condition.  Finally,  when  the 
material  has  in  this  manner  been  reduced  to  a  volume  of  about 
250  cc.,  it  is  transferred  to  a  clean,  dry  bottle,  which  is  tightly 
closed  to  protect  the  contents  from  the  air  and  to  prevent  loss 
of  water  by  evaporation. 

I.  DETERMINATION  OF  WATER 

Weigh  from  1  to  2  grams  of  the  powdered  coal  in  a  weighing 
glass  with  a  ground-glass  stopper  and  heat  in  an  air  bath  at  a 
temperature  of  105°  for  exactly  one  hour.  Allow  the  tube  to 
cool  in  a  calcium  chloride  desiccator  and  weigh  again.  The  loss 
in  weight  represents  the  water  mechanically  inclosed  in  the  coal, 
but  does  not  include  that  which  is  in  chemical  combination. 

II.  DETERMINATION  OF  THE  VOLATILE  COMBUSTIBLE  MATTER 

Weigh  a  fresh  quantity  of  the  coal,  ranging  from  1  to  2 
grams,  into  a  platinum  crucible.  Cover  the  crucible  and  heat 
exactly  3^  minutes  over  a  Bun  sen  burner,  and  then,  without 
giving  the  material  time  to  cool,  for  the  same  length  of  time  to 
the  highest  temperature  attainable  over  the  blast  lamp.  Place 
the  crucible,  while  still  warm,  in  a  calcium  chloride  desiccator. 
When  weighed  it  should  be  inclosed  in  a  weighing  glass  with  a 
ground-glass  stopper. 

The  difference  between  the  losses  (expressed  in  percentages) 
of  the  first  and  second  experiments  is  regarded  as  the  percentage 
of  volatile  combustible  matter  in  the  coal.  Since  the  coal  may 
contain  substances  having  water  in  chemical  combination,  which 
is  lost  at  the  temperature  of  the  blast  flame  but  not  below  105°, 
and  since  it  may  also  contain  carbonates  which  are  decomposed 
at  a  high  temperature,  it  will  be  seen  that  the  determination  of 
the  volatile  combustible  matter  by  this  method  is  somewhat 
uncertain. 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      377 

III.  DETERMINATION  OF  FIXED  CARBON 

Place  the  crucible  on  its  side  in  a  triangle  and  burn  out  the 
carbon,  taking  care  not  to  subject  the  contents  of  the  crucible  to 
unnecessarily  strong  draughts.  The  combustion  of  the  carbon 
may  be  considerably  hastened  by  stirring  the  material  occasion- 
ally with  a  platinum  spatula  or  wire.  The  loss  in  weight  is 
equal  to  the  fixed  carbon.  The  residue  of  noncombustible  mat- 
ter is  the  ash,  and  the  sum  of  the  ash  and  of  the  fixed  carbon 
is  the  coke  which  the  coal  will  yield. 


INCINERATION 

Incineration  is  the  name  given  to  the  process  of  destroying  by 
combustion  in  air  the  organic  matter  in  vegetable  and  animal 
substances,  with  a  view  to  the  determination  of  their  inorganic 
constituents  which  remain  after  such  treatment,  as  an  "  ash." 
The  usual  procedure  is  as  follows :  The  material  —  freed  from 
extraneous  matter  and  dried  under  such  conditions  as  may  be 
prescribed  by  the  nature  of  the  investigation  —  is  burned  in  a 
roomy  platinum  dish  in  a  muffle.  However  simple  the  process 
may  appear,  it  is  beset  with  many  difficulties,  some  of  which 
will  be  mentioned  together  with  the  customary  methods  of  meet- 
ing them.  The  ash  constituents  are  often  so  light  as  to  be 
carried  out  of  the  dish  by  air  currents ;  hence  it  is  necessary 
to  regulate  carefully  the  draught  through  the  muffle.  Again, 
the  combustion  must  be  effected  at  the  lowest  possible  tempera- 
ture. This  precaution  is  necessary  for  several  reasons :  (1)  If 
any  of  the  constituents  of  the  ash  are  fused,  the  unburned 
carbon  becomes  incrusted  with  liquid,  which  prevents  or  greatly 
retards  its  oxidation.  The  particular  substances  which  most 
frequently  give  rise  to  this  difficulty  are  the  salts  of  the  alkalies, 
and  it  is  often  necessary  in  the  case  of  materials  rich  in  these 
compounds  to  extract  the  partially  burned  matter  with  water 
before  the  incineration  can  be  completed.  (2)  At  a  full  red 


378  QUANTITATIVE  EXERCISES 

heat  the  alkalies  are  somewhat  volatile.  (3)  When  heated  to 
the  point  of  fusion  the  meta-  and  pyro-phosphates  decompose  the 
chlorides  and  sulphates  ;  hence  the  volatile  constituents  of  the 
latter  salts  may  be  lost.  Some  substances,  while  undergoing 
the  charring  process  which  precedes  the  actual  combustion, 
"  froth "  in  such  a  way  as  to  overflow  the  dish.  Materials 
behaving  in  this  manner  must  be  treated  in  small  portions  and 
with  great  caution  as  regards  the  regulation  of  the  temperature 
while  the  frothing  continues. 

In  the  case  of  substances  difficult  to  incinerate,  Rose  recom- 
mends the  mixing  of  the  charred  material  with  a  weighed  quantity 
of  platinum  sponge.  The  coal  obtained  by  the  preliminary 
heating  of  the  substance  is  transferred  to  a  porcelain  mortar 
and  cautiously  ground  to  a  fine  powder.  It  is  then  intimately 
mixed  with  30  grams  of  the  sponge  per  100  grams  of  the  original 
material.  The  mixture  is  placed,  a  small  portion  at  a  time, 
in  a  large  platinum  dish  and  very  gradually  heated  over  a  Bun- 
sen  burner  to  a  dark  red.  Before  the  temperature  of  the  mass 
has  risen  to  that  of  a  visible  red,  the  particles  of  carbon  at  the 
surface  begin  to  burn  and  the  black  mixture  becomes  coated 
with  a  gray  film.  The  combustion  is  hastened  by  constantly 
but  carefully  stirring  with  a  platinum  spatula.  The  glimmer- 
ing of  the  mass  continues,  as  fresh  portions  of  it  are  brought  to 
the  surface,  as  long  as  any  of  the  carbon  remains  unburned. 

Others  have  proposed  to  use  barium  carbonate,  barium  super- 
oxide,  ferric  oxide,  etc.,  instead  of  the  platinum  sponge  recom- 
mended by  Rose. 

Notwithstanding  the  most  painstaking  incineration  of  organic 
substances,  a  portion  of  the  chlorine,  sulphur,  and  phosphorus 
which  they  contain  may  be  lost  by  volatilization.  The  acid 
vapors  generated  during  the  charring  process  may  decompose 
the  chlorides  with  formation  of  hydrochloric  acid,  while  sulphur 
and  phosphorus,  in  so  far  as  they  are  constituents  of  organic 
compounds,  may  escape  in  an  unoxidized  or  only  partially  oxi- 
dized condition.  The  loss  of  chlorine  and  other  halogens  may 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      379 

be  prevented  and  the  loss  of  sulphur  and  phosphorus  may  be 
diminished  by  the  addition  of  strongly  basic  compounds,  such  as 
sodium  carbonate,  barium  hydroxide,  calcium  oxide,  etc.  Since, 
however,  the  use  of  such  substances  embarrasses  the  determina- 
tion of  other  ash  constituents,  the  chlorine,  sulphur,  and  phos- 
phorus are  usually  estimated  in  separate  portions  of  the  material 
and  by  special  methods. 

If  chlorine  is  to  be  determined,  about  10  grams  of  the  sub- 
stance, e.g.  vegetable  matter,  is  cut  or  crushed  until  it  is  reduced 
to  a  sufficiently  fine  condition  and  then  moistened  with  a  solu- 
tion containing  about  one  gram  of  sodium  carbonate.  The  liquid 
is  evaporated  and  the  residues  incinerated  at  the  lowest  practi- 
cable temperature.  When  the  burning  of  the  carbon  can  no 
longer  be  discerned,  the  soluble  matter  is  extracted  with  water 
and  the  insoluble  portion  again  incinerated. 

For  the  determination  of  sulphur  —  also  of  phosphorus  —  the 
material  in  a  state  of  fine  subdivision  is  treated  with  very  con- 
centrated nitric  acid  and  the  liquid  is  evaporated  to  dryness  on 
a  water  bath.  The  residue  is  moistened  with  the  acid  and  the 
evaporation  is  continued,  but  not  to  dryness.  Water  is  added 
and  then  2  or  3  grams  of  pure  anhydrous  sodium  carbonate. 
The  mass  is  again  dried  on  the  water  bath,  stirred  with  water  to 
the  consistency  of  a  thin  porridge,  and  thoroughly  mixed  with 
from  20  to  25  grams  of  pulverized  sodium  carbonate.  The 
mixture  is  dried,  ground  to  a  fine  powder,  and  heated  over  an 
alcohol  lamp  —  but  not  to  the  point  of  fusion  —  in  a  silver  or 
platinum  dish  until  it  becomes  perfectly  white.  If,  on  continued 
heating,  the  mass  fails  to  lose  its  color,  it  is  ground,  mixed  with 
a  little  potassium  nitrate,  and  again  heated  as  before.  In  this 
manner  the  sulphur  in  the  material  is  converted  into  sulphates 
and  the  phosphorus  into  phosphates.  This  method  is  known  as 
that  of  Knop  and  Arendt. 


380  QUANTITATIVE  EXERCISES 

DETERMINATION  OF  ORGANIC  MATTER  IN  WATER 

The  presence  in  waters  of  appreciable  quantities  of  organic 
matter  of  vegetable  or  animal  origin  is  regarded  as  an  element 
of  the  highest  importance  in  determining  their  fitness  for 
domestic  uses.  But  since  it  is  in  general  wholly  impracticable 
to  isolate,  or  even  to  identify,  the  individual  organic  compounds 
in  any  specimen  of  well  or  spring  water,  indirect  methods  are 
resorted  to  which  have  for  their  object  the  testing  of  the  proba- 
bility and  approximately  the  extent  of  such  contamination. 

One  of  these  methods,  the  "Albuminoid  Ammonia  Process  " 
of  Wanklyn,  Chapman,  and  Smith,  has  already  been  presented 
in  the  chapter  on  the  determination  of  nitrogen.  The  applica- 
tion of  it  to  the  question  in  hand  depends  upon  the  fact  that 
most  nitrogenous  organic  compounds  of  animal  and  vegetable 
origin  are  decomposed  by  potassium  permanganate  in  the  pres- 
ence of  an  alkali  in  such  a  manner  that  the  nitrogen  is  to  a 
large  extent  converted  into  ammonia.  The  same  nitrogenous 
compounds  yield  ammonia  and  nitrous  and  nitric  acids  when 
decomposed  by  fermentative  processes ;  hence  the  presence  of 
the  latter  substances  in  a  water  is  regarded  as  an  evidence  of 
previous  contamination,  and  the  quantity  of  them  is  regarded 
as  an  approximate  measure  of  pollution.  In  a  similar  manner 
great  weight  is  attached  to  the  presence  of  unusual  quantities 
of  chlorine  in  a  water,  because  sodium  chloride  is  an  abundant 
constituent  of  urine  and  of  other  forms,  of  animal  waste  which 
are  likely  to  find  their  way  into  the  water  courses  both  above 
and  below  the  surface  of  the  ground. 

There  are  numerous  processes  for  the  approximate  estimation 
of  organic  matter  in  water,  which  depend  upon  the  fact  that  the 
organic  compounds  always  present  in  polluted  waters  readily 
reduce  potassium  permanganate.  Such  methods  are  expeditious, 
but  they  cannot  afford  very  precise  information  as  to  the  total 
quantity  of  the  contaminating  matter,  since  different  compounds 
are  oxidized  to  different  degrees  by  potassium  permanganate,  and 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL     381 

since  the  extent  to  which  any  given  compound  will  be  oxidized 
will  depend  very  much  upon  the  length  of  time  allowed  for  the 
reaction  and  also  upon  the  temperature  of  the  water.  Some  of 
the  processes  involving  the  use  of  potassium  permanganate  are 
given  here. 

THE  METHOD  OF  KUBEL 

One  hundred  cubic  centimeters  of  the  water  to  be  tested  are 
treated  in  a  300-cc.  flask  with  5  cc.  of  dilute  sulphuric  acid 
(1  part  of  strong  acid  to  3  parts  of  water),  and  with  enough  of 
a  standard  solution  of  potassium  permanganate  (equivalent  to 
jl-g-  normal  oxalic  acid)  to  produce  a  deep  red  color.  The  solu- 
tion is  boiled  for  10  minutes  and  then  treated  with  10  cc.  of 
Y^-Q-  normal  oxalic  acid.  The  solution,  which  becomes  colorless 
on  the  addition  of  the  oxalic  acid,  is  titrated  to  a  faint  red  color 
with  the  standard  permanganate.  The  difference  between  the 
total  quantity  of  permanganate  used  arid  10,  the  volume  of  the 
oxalic  acid  which  was  added,  is  the  volume  of  the  permanga- 
nate reduced  by  the  organic  matter  in  100  cc.  of  the  water.  It 
is  customary  to  state  the  results  in  milligrams  of  oxygen  con- 
sumed by  100  cc.,  i.e.  in  parts  per  100,000. 

If  nitrites  are  present,  a  portion  of  the  oxygen  is  employed 
in  converting  them  into  nitrates ;  but  the  presence  of  nitrites 
is  in  itself  a  certain  evidence  of  contamination. 

THE  METHOD  OF  SCHULZE 

One  hundred  cubic  centimeters  of  the  water  are  treated  with 
0.5  cc.  of  caustic  soda  (1  part  of  the  hydroxide  to  2  parts  of  water), 
and  with  10  cc.  —  or  15  in  the  case  of  very  impure  water  —  of 
a  standard  potassium  permanganate  (equivalent  to  y^  normal 
oxalic  acid).  The  water  is  boiled  for  about  10  minutes,  allowed  to 
cool  to  60°  or  even  50°,  and  treated  with  5  cc.  of  dilute  sulphuric 
acid  and  10  cc.  of  T^-g-  normal  oxalic  acid.  The  excess  of  oxalic 
acid  is  then  determined  by  the  standard  permanganate,  as  in  the 
method  of  Kubel,  and  the  result  is  stated  in  the  same  manner. 


382  QUANTITATIVE  EXERCISES 

THE  METHOD  OF  TIDY 

This  process  differs  from  both  of  the  preceding  ones  in  that 
the  action  of  the  permanganate  on  the  organic  matter  is  allowed 
to  take  place  at  the  ordinary  temperatures,  and  also  in  the 
manner  of  determining  the  excess  of  the  permanganate.  A 
measured  quantity  of  the  water  (250  cc.)  is  treated  with  dilute 
sulphuric  acid  (10  cc.)  and  a  definite  volume  (10  cc.)  of  a  dilute 
standard  solution  of  permanganate.  After  standing  for  3  hours 
at  the  temperature  of  the  room,  a  small  quantity  of  a  solution 
of  potassium  iodide  is  added  to  the  water,  and  the  iodine  which 
is  liberated  is  determined  by  means  of  a  standard  solution  of 
sodium  thiosulphate. 

In  connection  with  the  permanganate  processes  should  be 
mentioned  the  method  of  Fleck,  who  estimates  the  organic  mat- 
ter in  polluted  water  by  means  of  its  reducing  action  upon  an 
alkaline  silver  solution.  To  prepare  the  standard  silver  solu- 
tion, 17  grams  of  silver  nitrate  are  dissolved  in  a  little  water. 
This  solution  is  then  added  to  another  containing  50  grams  of 
sodium  thiosulphate  and  48  grams  of  sodium  hydroxide.  The 
mixed  solution  is  boiled  for  10  or  15  minutes,  to  destroy  any 
organic  matter  in  the  water,  and  allowed  to  settle.  To  deter- 
mine its  strength,  a  measured  quantity  of  the  liquid  is  titrated 
with  a  -£Q  normal  solution  of  potassium  iodide  until  a  drop  taken 
out  with  a  glass  rod  and  placed  upon  a  white  porcelain  surface 
gives  a  blue  color  with  a  drop  of  a  chromic  acid  and  starch  solu- 
tion. The  last  reagent  is  prepared  by  mixing  equal  volumes 
of  a  clear  starch  solution,  a  solution  of  potassium  bichromate 
(1 :  20),  and  concentrated  hydrochloric  acid. 

The  determination  is  made  in  the  following  manner  :  100  cc. 
of  the  water  under  examination  are  treated  with  10  cc.  of  the 
standard  silver  solution  and  boiled  for  about  10  minutes,  or 
until  the  precipitate  which  is  at  first  finely  divided  becomes 
more  compact  and  exhibits  a  tendency  towards  rapid  subsidence. 
When  cold  the  excess  of  the  silver,  i.e.  that  which  remains  in 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      383 

solution,  is  determined  by  means  of  the  standard  potassium 
iodide  solution,  with  use  of  the  solution  of  starch  and  chromic 
acid  as  the  indicator.  The  result  is  stated  in  milligrams  of  silver 
reduced  by  the  organic  matter  in  100  cc.  of  water,  i.e.  in  parts 
per  100,000. 

There  are  also  several  methods  by  means  of  which  may  be 
made  in  impure  waters  a  more  or  less  accurate  quantitative 
estimation  of  the  organic  carbon  or  of  the  organic  carbon  and 
nitrogen.  The  failure  of  all  of  them  to  give  entirely  accurate 
results  is  due  to  the  fact  that  a  portion  of  the  organic  matter 
in  polluted  waters  is  lost  by  volatilization  during  the  evapora- 
tion which  necessarily  precedes  the  combustion  of  the  carbon 
compounds.  A  brief  summary  of  the  best  of  the  methods  of 
this  class  is  here  given. 

THE  METHOD  OF  WOLFF,  DEGENER,  AND  HERGFELD 

In  a  retort  over  the  free  flame  500  or  1000  cc.  of  the  water 
are  evaporated  to  about  one-half  the  original  volume.  During 
the  distillation  the  retort  is  connected  with  a  condenser  and  a 
receiver.  The  distillate  is  examined  by  one  of  the  perman- 
ganate methods  for  an  approximate  estimation  of  the  volatile 
organic  matter.  The  liquid  remaining  in  the  retort  is  trans- 
ferred to  a  platinum  or  glass  dish  and  further  evaporated  upon 
a  water  bath  to  a  volume  of  about  15  cc.  The  concentrated 
liquid  is  then  transferred  to  a  250-  or  300-cc.  flask.  The 
retort  and  also  the  dish  in  which  the  water  was  last  evaporated 
are  washed  with  a  small  quantity  (10  cc.)  of  dilute  sulphuric 
acid,  and  the  washings  are  poured  into  the  flask.  The  carbon 
dioxide  liberated  by  the  sulphuric  acid  is  removed  by  warming 
the  liquid  to  about  50°  and  by  conducting  a  current  of  air 
through  the  flask.  The  flask  is  provided  with  a  stopper  carry- 
ing a  tube  bent  to  an  obtuse  angle  for  the  exit  of  the  carbon 
dioxide,  a  small  separating  funnel  for  the  introduction  of  sul- 
phuric acid  and  air,  and  a  thermometer.  The  exit  tube  is 


384  QUANTITATIVE  EXERCISES 

connected  with  an  apparatus  for  the  purification  and  absorp- 
tion of  the  carbon  dioxide.  This  consists  (1)  of  a  condenser 
inclined  in  an  upward  direction  from  the  flask,  (2)  a  U-tube 
filled  with  calcium  chloride  for  the  absorption  of  water,  (3)  a 
U-tube  containing  coarsely  powdered  antimony  for  the  absorp- 
tion of  chlorine,  (4)  another  U-tube  filled  with  calcium  chlo- 
ride, (5)  the  usual  weighed  absorption  apparatus  for  carbon 
dioxide,  together  with  a  tube  containing  calcium  chloride  and 
soda-lime  for  its  protection  against  the  water  and  carbonic  acid 
of  the  air. 

To  the  contents  of  the  flask  are  added  10  grams  of  finely 
powdered  potassium  bichromate.  There  are  then  introduced 
through  the  separating  funnel  50  or  60  cc.  of  sulphuric  acid,  — 
3  volumes  of  the  concentrated  acid  diluted  with  2  volumes  of 
water.  For  a  half  hour  the  temperature  of  the  liquid  in  the 
flask  is  maintained  at  50°  or  55°.  During  the  second  half  hour 
it  is  gradually  raised  to  the  boiling  point.  After  boiling  for 
about  10  minutes,  air  freed  from  carbon  dioxide  is  aspirated 
through  the  apparatus. 

THE  METHOD  OF  DUPRE  AND  HAKE 

A  convenient  quantity  of  the  water  is  acidified  with  phos- 
phoric acid  to  decompose  carbonates,  and  evaporated  to  dryness. 
The  residue  is  transferred  to  a  platinum  boat  and  burned  in  a 
current  of  oxygen  in  a  glass  tube  which  is  properly  filled  for 
the  combustion  of  organic  compounds  containing  nitrogen.  The 
carbon  dioxide  developed  during  the  combustion  is  collected  in 
a  solution  of  barium  hydroxide.  The  barium  carbonate  is  col- 
lected on  a  filter,  washed,  and  dissolved  in  dilute  hydrochloric 
acid.  The  barium  in  the  filtrate  is  determined  as  sulphate  in 
the  usual  manner. 

The  same  authors  have  also  proposed  a  nephelometric  method 
for  the  determination  of  minute  quantities  of  carbon  in  water. 
It  depends  upon  the  fact  that  the  turbidity  produced  by  the 


DETERMINATION  OF  CARBON  IN  IRON  AND  STEEL      385 

precipitation  of  a  small  quantity  of  lead  carbonate  from  a  dilute 
solution  of  lead  acetate  may,  with  the  aid  of  suitable  standards, 
be  utilized  for  the  estimation  of  the  carbonic  acid. 


THE  METHOD  OF  DITTMAR  AND  KOBINSON 
1.  Determination  of  Organic  Carbon 

A  suitable  quantity  of  the  water  is,  treated  with  T -fo^  of  its 
volume  of  a  saturated  solution  of  sulphurous  acid  to  decompose 
any  carbonates  which  may  be  present,  and  evaporated  to  a  small 
volume  in  a  flask  of  the  Erlenmeyer  form,  and  afterwards  to 
dryness  in  a  glass  dish.  The  residue  is  transferred  to  a  plati- 
num boat  and  burned  with  the  aid  of  a  current  of  air  in  a  com- 
bustion tube  which  is  filled  with  copper  oxide,  etc.,  in  the  usual 
manner.  The  apparatus  for  the  purification  of  the  carbon 
dioxide,  previous  to  absorption,  differs  from  the  one  ordinarily 
used  in  that  a  tube  containing  a  solution  of  chromic  acid  in 
60  per  cent  sulphuric  acid  is  inserted  between  the  combustion 
and  the  calcium  chloride  tubes  for  the  purpose  of  intercepting 
sulphur  dioxide. 

2.  Determination  of  Organic  Nitrogen 

Half  a  liter  of  water  is  evaporated  to  a  small  volume  in  a 
flask  which  is  connected  with  a  condenser  and  receiver.  The 
ammonia  in  the  distillate  is  determined  colorimetrically.  To 
the  liquid  remaining  in  the  flask  are  added  sulphurous  acid 
and  ferric  chloride  to  destroy  nitrates.  The  solution  is  gently 
heated,  then  treated  with  a  little  sodium  sulphite,  to  neutralize 
any  sulphuric  acid  which  may  have  been  formed,  transferred  to 
a  glass  or  platinum  dish,  and  evaporated  to  dryness  upon  a 
water  bath.  The  nitrogen  in  the  residue  is  converted  by  the 
method  of  Varrentrapp  and  Will  into  ammonia,  which  is  deter- 
mined colorimetrically. 


386  QUANTITATIVE  EXERCISES 

THE  METHOD  OF  FRANKLAND  AND  ARMSTRONG 
Determination  of  Organic  Carbon  and  Nitrogen 

As  regards  the  estimation  of  the  characteristic  constituents 
of  the  organic  compounds  in  impure  waters,  this  process  is  prob- 
ably more  exact  than  any  of  those  hitherto  mentioned.  But 
since  it  fails  to  account  for  the  carbon  and  nitrogen  which  are 
lost  during  evaporation,  it  is  not  an  entirely  accurate  method. 
It  is  very  complicated  ;  hence  only  the  outlines  of  the  process 
can  be  given  here. 

The  water  is  treated  with  sulphurous  acid  and  a  small  quan- 
tity of  ferric  chloride,  to  eliminate  carbonic,  nitric,  and  nitrous 
acids,  and  evaporated  to  dry  ness  in  a  bath  of  special  construc- 
tion. The  residue  is  mixed  with  copper  oxide  and  transferred 
to  a  combustion  tube.  More  of  the  oxide  and  also  a  quantity  of 
metallic  copper  are  introduced.  The  tube  is  drawn  out,  attached 
to  a  Sprengel  pump,  and  exhausted.  The  substance  is  burned, 
and  the  gaseous  products  of  the  combustion  are  collected 
over  mercury  and  determined  by  gasometric  methods.  The 
substances  to  be  estimated  are  carbon  dioxide,  nitrogen,  and 
usually  a  small  quantity  of  nitric  oxide.  From  the  total  quan- 
tity of  nitrogen  found  must  be  deducted  that  which  was  derived 
from  ammonia  and  ammonium  salts  ;  and  to  ascertain  the  mag- 
nitude of  this  correction,  a  determination  of  ammonia  in  another 
portion  of  the  water  must  be  made. 


CHAPTER  XVII 
GAS  ANALYSIS 

The  room  in  which  gas-analytical  operations  are  performed 
should  be  so  located  that  an  even  temperature  can  be  main- 
tained for  a  considerable  length  of  time.  It  should  also  have 
a  mercury-tight  floor.  If  this  is  not  practicable,  the  floor 
under  the  work  table  and  for  some  distance  around  it  should 
be  covered  with  a  single  piece  of  linoleum,  the  edges  of  which 
are  raised  to  prevent  the  escape  of  mercury. 

The  table  should  have  a  mercury-tight  top  and  a  raised 
border,  and  should  be  level.  At  one  corner,  or  some  other 
convenient  place,  it  is  to  be  provided  with  a  hole  for  the  escape 
of  mercury  into  a  receptacle  placed  beneath.  The  vessel  used 
to  receive  the  mercury  which  is  spilled  from  time  to  time  upon 
the  table  should  be  covered  to  protect  the  contents  from  the 
dust  of  the  air,  and  should  be  provided  with  a  filter  through 
which  the  metal  must  pass  before  entering  it.  Fastened  to  the 
table  top,  near  one  end,  is  a  wooden  standard  a  little  more  than 
a  meter  in  height.  This  is  provided  with  a  movable  wooden 
arm  having  a  set-screw  by  which  the  arm  may  be  firmly  fixed 
in  any  desired  position.  The  purpose  of  the  arm  is  to  hold  the 
eudiometer  in  place  during  an  explosion  of  its  contents.  A 
cork,  with  one  end  hollowed  out  to  fit  the  closed  end  of  the  tube, 
is  placed  between  the  wooden  arm  and  the  eudiometer  whenever 
an  explosion  is  to  be  made. 

The  wooden  mercury  trough  with  sides  of  plate  glass,  which 
is  usually  employed  in  gas  analysis,  is  shown  in  Fig.  56.  It 
rests  upon  a  board,  at  one  end  of  which  are  two  standards  sur- 
mounted by  an  inclined  grooved  piece  which  is  lined  with  felt 
and  serves  as  a  rest  for  the  eudiometer.  Before  using  the 

387 


388  QUANTITATIVE  EXERCISES 

trough  the  interior  is  moistened  with  a  solution  of  mercuric 
chloride,  and  then  rubbed  till  dry  with  mercury.  This  treat- 
ment causes  the  mercury  to  adhere  to  the  wood  and  thus  pre- 
vents the  lodgment  of  air 
between  them.  A  rest  for 
the  eudiometer,  when'  an 
explosion  is  to  be  made,  is 
prepared  by  shaping  one  end 
of  a  cork  to  fit  the  bottom 
of  the  trough  and  covering 
the  other  end  with  a  piece 
of  sheet  rubber,  which  is 

stretched  over  the  top  and 
FIG.  56  iiiii 

tacked  to  the  sides  with 

steel  brads.     The  rubber  covering  of  the  cork,  like  the  bottom 
of  the  trough,  is  rubbed  with  mercuric  chloride  and  mercury. 

There  are  also  required  in  a  room  devoted  to  the  analysis  of 
gases : 

1.  A  telescope  for  reading  the  eudiometer  at  a  distance. 

2.  An  induction  coil  with  wires  running  to  the  work  table. 

3.  A  battery  consisting  of  four  bichromate  cells,  or  three 
storage  cells,  with  wires  running  to  the  induction  coil. 

4.  A  barometer  —  usually  of  the  siphon  variety  —  which  is 
located  so  as  to  be  read  by  the  telescope  without  moving  the 
standard  of  the  latter  from  its  proper  position  for  reading  the 
eudiometer.     In  the  open  end  of  the  barometer  is  placed  a  cork, 
with  vertically  grooved  sides,  which  carries  a  loosely  fitting  ther- 
mometer.    Some  time  before  reading  the  barometer  the  mercury 
in  it  is  set  in  motion  by  lowering  and  raising  the  thermometer 
through  the  cork.    . 

5.  Two  plumb  lines  to  aid  in  bringing  the  eudiometer  into  a 
vertical  position.     These  are  suspended  from  the  ceiling  near 
the  middle  of  adjacent  sides  of  the  room. 

6.  A  Bun  sen  apparatus  for  the  generation  of  electrolytic  gas, 
Fig.  57.     The  inner  vessel  contains  dilute  sulphuric  acid  (1  part 


GAS  ANALYSIS 


389 


FIG.  57 


of  the  concentrated  acid  to  10  parts  of  water),  while  the  outer 
one  is  filled  with  water  (or,  if  there  is  danger  of  freezing,  with 
alcohol)  to  prevent  the  heating  of  the 
wires  by  the  current.  The  enlargements 
in  the  upper  part  of  the  exit  tube  are 
made  to  hold  a  small  quantity  of  concen- 
trated sulphuric  acid  when  it  is  desired 
to  introduce  the  electrolytic  gas  into  the 
eudiometer  in  a  dry  condition.  Before 
using  the  gas  it  must,  of  course,  be  allowed 
to  waste  until  the  air  has  been  expelled 
from  the  acid  and  the  exit  tube. 

7.  A  Bunsen  apparatus  for  the  gener- 
ation of  pure  hydrogen,  Fig.  58.  In  this 
the  positive  pole  is  located  in  the  bottom 

of  the  inner  cylinder,  where  the  platinum  wire,  after  passing 
through  the  glass,  is  submerged  in  mercury  in  which  a  little 
pure  zinc  is  dissolved  from  time  to  time. 
The  oxygen  which  appears  at  this  pole  is 
disposed  of  in  the  following  manner : 

H2SO4  +  Zn  =  ZnSO4  +  2  H, 
2H  +  0  =  H20. 

The  zinc  in  the  amalgam  is  scarcely  at- 
tacked  by  the  dilute  sulphuric  acid  except 
while  the  circuit  is  closed.  The  liquid 
immediately  above  the  positive  pole  soon 
becomes  saturated  with  zinc  sulphate, 
which  prevents  the  free  diffusion  of  fresh 
acid  to  the  zinc  amalgam;  for  this  reason  the  side  tube  rep- 
resented in  the  figure  is  added  to  the  apparatus.  After  each 
experiment  the  exit  tube  for  gas  is  removed  and  fresh  acid  is 
poured  in  at  the  top,  while  the  concentrated  solution  of  zinc  sul- 
phate in  the  bottom  of  the  cylinder  is  allowed  to  escape  through 
the  side  tube.  The  acid  employed  for  the  generation  of  pure 


FIG.  58 


390  QUANTITATIVE  EXERCISES 

hydrogen,  like  that  used  in  making  electrolytic  gas,  is  prepared 
by  diluting  1  part  of  concentrated  sulphuric  acid  with  10  parts 
of  water. 

The  calibration  of  the  eudiometer  which  is  to  be  used  in  the 
following  experiments,  and  also  the  reduction  of  the  volumes  of 
gases  which  are  measured  in  it,  have  been  fully  explained  in  a 
previous  chapter.  The  eudiometer  should  have  a  length  of 
about  760  mm.,  though  shorter  or  longer  ones  may  be  used. 

When  a  eudiometer  is  to  be  filled,  previous  to  the  collection 
of  a  gas  in  it,  it  is  well  to  lay  the  tube  in  an  inclined  wooden 
trough  which  is  lined  with  felt,  and  to  introduce  the  mercury 
very  slowly  through  a  glass  tube  which  reaches  to  the  bottom 
of  the  eudiometer.  Otherwise  air  may  become  entangled  between 
the  mercury  and  the  glass,  and  to  remove  this  often  causes 
much  trouble.  A  good  arrangement  for  filling  with  mercury  is 
made  by  attaching  the  long  tube  which  enters  the  eudiometer 
to  the  stem  of  a  separating  funnel  with  a  stopcock,  the  stem  of 
the  funnel  having  been  bent  to  the  proper  angle. 

The  eudiometer  having  been  filled  and  inverted  with  its  open 
end  dipping  under  the  mercury  in  the  trough,  it  may  be  fixed 
in  a  vertical  position  or  laid  in  the  inclined  grooved  piece  for 
the  introduction  of  gas.  In  ascending,  small  portions  of  the 
gas  sometimes  become  lodged  on  the  side  of  the  eudiometer. 
To  remove  these,  the  tube  is  tilted  to  one  side  or  laid  in  the 
receptacle  at  the  end  of  the  trough,  and  by  raising  and  lowering 
it  the  mercury  column  is  made  to  ascend  and  descend,  but  in 
such  a  manner  that  the  upward  movements  are  more  rapid  than 
those  in  the  downward  direction.  Having  introduced  the  gas, 
the  eudiometer  is  brought  to  a  vertical  position  with  the  aid 
of  the  plumb  lines,  and  fastened  in  the  clamp  of  an  iron  stand. 
The  operator  should  then  set  the  mercury  in  the  barometer  in 
motion  and  retire  to  a  distance  until  the  gas  in  the  tube  has  had 
time  to  take  on  the  temperature  of  the  room,  which  it  will  do 
all  the  more  quickly  if  he  has  taken  the  precaution  to  use  cold 
gloves  or  cloths  in  handling  the  eudiometer. 


GAS  ANALYSIS  391 

The  readings  to  be  made  are: 

1.  The  height  of  the  barometer  and  its  temperature. 

2.  The  volume  of  the  gas. 

3.  The  height  of  the  column  of  mercury  in  the  eudiometer 
above  the  level  of  the  mercury  in  the  trough. 

4.  The  temperature  as  indicated  by  a  thermometer  whose  bulb 
is  buried  in  the  mercury  near  the  lower  end  of  the  eudiometer. 

It  is  often  necessary  in  gas  analysis  to  mix  gases  in  eudi- 
ometers in  certain  definite  proportions ;  in  other  words,  having 
collected  and  measured  one  gas,  to  introduce  another  until  the 
volume  of  the  second,  if  both  were  measured  under  standard 
pressure,  bears  a  predetermined  ratio  to  the  volume  of  the  first. 
But,  since  the  first  gas  and  the  mixture  are  measured  under 
different  pressures,  the  volume  of  the  first,  as  read  upon  the 
millimeter  scale,  gives  no  very  obvious  clue  as  to  when  the 
introduction  of  the  second  is  to  be  discontinued.  It  has  been 
customary,  in  accordance  with  the  recommendation  of  Bunsen, 
to  prepare  a  table  for  each  eudiometer  which  should  show  in 
a  general  way  by  actual  trial  how  much  space  equal  volumes 
of  gas  will  occupy  in  different  parts  of  the  tube,  and  from ' 
which  the  data  required  in  any  given  case  could  be  approxi- 
mately obtained  by  a  process  of  interpolation. 

The  difficulty  is  obviated  by  considering  how  the  tensions  of  the 
two  gases,  i.e.  that  of  the  first  and  of  the  mixture  made  by  adding 
a  definite  amount  of  a  second  gas,  are  related  to  each  other. 
Let  v  =  the  volume  of  the  first  gas  as  measured  in  scale  divisions. 
£  =  its  tension,  i.e.  the  difference  between  the  height  of 
the  barometer  and  the  height  of  the  mercury  in  the 
eudiometer. 

<j  =  the  tension  of  the  mixed  gases,  i.e.  the  difference 
between  the  height  of  the  barometer  and  the  height  of 
the  mercury  in  the  eudiometer  after  the  introduction 
of  the  second  gas. 

1^  =  the  volume  of  the  mixed  gases  as  measured  in  scale 
divisions. 


392  QUANTITATIVE  EXERCISES 

Then     v1=v  +  tl-t.  (1) 

Let  K  —  the  quantity  of  the  mixture  as  a  multiple  of  the 
quantity  of  the  first  gas,  e.g.  if  the  tube  contains  air 
and  1T2  is  to  be  introduced,  then  K  will  be  1.5,  2, 
2.5,  or  3,  according  as  the  quantity  of  H^  is  to  be  0.5, 
1,  1.5,  or  2  times  as  great  as  that  of  the  air. 

Then     JT=^. 
vt 

Clearing  of  fractions, 

Kvt  =  v^.  (2) 

Multiplying  equation  (1)  by  tv  we  have 

v1t1  =  t1(v-}-tl-t)9  (3) 

and  substituting  Kvt  for  v^  in  (3),  we  have 

Kvt  =  tl(v  +  t1-t).  (4) 

Finding  the  value  of  ^  in  (4),  we  have 


_  _  (y  -  t)  +        (fl  -  t)2  +  4  Kvt 

tl~  ~T~ 

and  the  height  of  the  barometer,  less  t^  equals  the  height  of  the 
mercury  in  the  eudiometer  after  the  introduction  of  the  second 
gas,  i.e.  to  what  point  on  the  scale  the  mercury  will  be  depressed. 
By  assigning  the  proper  fractional  values  to  K,  one  can  deter- 
mine with  equal  facility  where  the  mercury  meniscus  will  stand 
if  definite  portions  of  a  gas  are  removed  from  a  eudiometer, 
e.g.  if  0.25,  0.50,  or  0.75  of  the  gas  is  to  be  withdrawn,  the 
value  of  K  will  be  0.75,  0.5,  or  0.25. 

EXERCISE  XLIII 

DETERMINATION  OF  OXYGEN  IN  THE  AIR 
I.    BY  EXPLOSION  WITH  HYDROGEN 

When  a  mixture  of  oxygen  and  hydrogen  is  exploded  in 
the  presence  of  nitrogen,  as  in  the  determination  of  oxygen  in 
the  air,  the  proportion  of  the  explosive  gases  (the  oxygen  plus 
twice  its  volume  of  hydrogen)  to  the  nonexplosive  gases  (the 


GAS  ANALYSIS  393 

nitrogen  plus  the  excess  of  hydrogen)  must  not  exceed  the  ratio 
of  64  to  100.  Otherwise  there  is  danger  that  a  portion  of  the 
oxygen  will  combine  with  the  nitrogen.  On  the  other  hand, 
the  proportion  of  the  explosive  gases  to  the  nonexplosive  must 
not  fall  below  the  ratio  of  26  to  100,  since,  in  that  case,  not  all 
of  the  oxygen  will  combine  with  hydrogen. 

It  often  happens,  in  introducing  hydrogen  for  the  purpose  of 
removing  oxygen,  that  too  large  a  quantity  of  the  gas  is  allowed 
to  enter  the  tube,  and  no  explosion,  or  only  a  weak  one,  follows 
the  passage  of  the  spark.  In  such  cases  it  is  necessary  to 
increase  the  proportion  of  explosive  gases  by  introducing  elec- 
trolytic gas,  care  being  taken,  of  course,  not  to  exceed  the  limit 
already  given.  The  volume  of  the  electrolytic  gas  so  introduced 
need  not  be  measured,  since  it  all  disappears  on  explosion. 

Place  a  minute  drop  of  water  in  the  closed  end  of  the  eudi- 
ometer, or  moisten  the  inner  wall  near  the  top  with  a  damp 
cloth  on  the  end  of  a  rod.  Rest  the  tube  in  the  inclined  rack 
previously  mentioned,  fill  with  mercury,  and  transfer  it  to  the 
trough.  The  air  which  is  to  be  analyzed  must  be  freed  from 
carbon  dioxide,  since,  if  the  latter  gas  is  present  at  the  time  of 
the  explosion,  it  will  be  reduced  to  carbon  monoxide  by  the 
excess  of  hydrogen.  For  the  collection  of  the  air  and  its  intro- 
duction into  the  eudiometer,  bulbs  with  two  short  and  rather 
narrow  stems  on  opposite  sides  are  to  be  recommended.  These 
can  be  readily  made  by  blowing  bulbs  in  the  middle  of  glass 
tubes  and  cutting  off  the  ends.  They  need  not  have  a  capacity 
of  more  than  15  or  20  cc.,  even  when  the  determination  is  to 
be  duplicated.  One  end  is  joined  by  a  short  piece  of  rubber  to 
a  narrow  tube  which  has  been  bent  to  a  shape  convenient  for 
the  introduction  of  the  gas  into  the  eudiometer,  while  the  other 
end  is  connected  by  a  rubber  tube  of  some  length  to  a  second 
bulb  of  the  same  kind  which  serves  as  a  reservoir.  The  bulb 
which  is  to  hold  the  air  is  filled  with  mercury,  and  a  tube  contain- 
ing a  little  sodium  hydroxide  and  a  small  quantity  of  calcium 
chloride  to  absorb  carbon  dioxide  and  ammonia  is  attached  to  the 


394  QUANTITATIVE  EXERCISES 

exit,  and  the  reservoir  slowly  lowered.  The  absorption  tube  is 
detached  and  the  air  introduced  into  the  eudiometer  by  raising 
the  reservoir.  The  amount  to  be  introduced  must  be  deter- 
mined by  the  judgment  of  the  experimenter.  When  the  eudi- 
ometer is  in  an  upright  position,  the  air  will,  of  course,  be  much 
expanded  (especially  if  the  tube  is  a  long  one),  and  the  quantity 
of  it  will  seem  to  be  more  than  it  really  is. 

Having  introduced  the  air,  fix  the  eudiometer  in  a  vertical 
position  with  the  aid  of  the  plumb  lines,  set  the  mercury  in  the 
barometer  in  motion,  and  retire  until  the  gas  has  had  ample 
time  to  take  on  the  temperature  of  the  room.  Read  with  the 
telescope  the  volume  of  the  gas,  the  height  of  the  column  in 
the  tube  from  the  level  of  the  mercury  in  the  trough  to  the 
top  of  the  meniscus,  and  the  height  of  the  barometer  and  of 
the  thermometers. 

Introduce,  without  moving  the  eudiometer,  a  quantity  of 
hydrogen  equal  to  that  of  the  air,  using  the  formula  given  on 
a  previous  page  to  determine  how  far  the  mercury  column  will 
be  depressed  when,  as  in  this  case,  K  equals  2.  Tilt  the  eudi- 
ometer to  one  side  and  mix  the  gases  thoroughly  by  quickly  rais- 
ing and  lowering  the  tube  in  such  a  way  as  to  set  the  mercury 
column  in  motion.  Restore  the  eudiometer  to  its  vertical  posi- 
tion, allow  the  mixed  gases  to  cool,  and  then  make  the  same  read- 
ings as  before.  Rest  the  eudiometer  upon  the  rubber-covered  cork, 
place  the  cork  with  one  concave  end  upon  the  top,  and  bring  the 
wooden  arm  down  upon  it  with  just  enough  pressure  to  hold 
the  tube  firmly  in  place.  Explode,  remove  the  corks,  and  after 
the  contents  of  the  eudiometer  have  cooled,  read  again.  The 
percentage  by  volume  of  the  oxygen  in  the  air  is  about  20.9. 

If  the  explosion  is  a  weak  one,  there  is  a  chance  that  the 
proportion  of  explosive  gases  was  too  small,  and  a  quantity  of 
the  electrolytic  gas  must  be  introduced,  the  contents  of  the 
tube  well  mixed,  and  the  explosion  repeated. 

When  the  quantity  of  the  oxygen  in  a  gas  mixture  is  not 
even  approximately  known,  one  begins  by  adding  an  amount  of 


GAS  ANALYSIS 


395 


hydrogen  so  large  as  to  remove  all  danger  of  combination  of 
oxygen  with  nitrogen  or  of  shattering  the  tube.  If  then,  as 
usually  happens,  no  explosion  or  only  a  feeble  one  is  observed, 
the  proportion  of  explosive  gases  is  increased  by  the  addition 
of  electrolytic  gas  until  the  volume  of  the  gas  in  the  eudiometer 
is  found  to  remain  constant  after  successive  explosions. 

The  volume  of  the  water  which  is  formed  may  be  neglected 
in  ordinary  eudiometric  experiments,  since  it  amounts  only  to 
0.00092  of  the  volume  of  the  gases  (under  standard  conditions) 
which  disappear  during  the  explosion. 


II.    BY  ABSORPTION  WITH  PHOSPHORUS 

Under  ordinary  pressure  pure  oxygen  is  not  absorbed  by 
phosphorus.  If,  however,  the  oxygen  is  diluted  with  another 
gas,  or  if  the  pressure  is  reduced  to  about  three-fourths  of  an 
atmosphere  or  less,  absorption  takes  place.  The  reac- 
tion is  prevented  by  the  presence  of  various  sub- 
stances. Among  these  are  certain  hydrocarbons  (such 
as  ethylene),  ammonia,  and  the  vapors  of  ethereal  oils 
and  of  alcohol.  It  is  also  prevented,  or  very  much 
retarded,  by  low  temperatures.  For  all  gas  analytical 
purposes  a  temperature  not  much  below  20°  should 
be  maintained  during  the  absorption.  The  formation 
of  ozone  may,  in  certain  instances,  render  imprac- 
ticable the  use  of  phosphorus  as  an  absorbent  of 
oxygen.  This  is  the  case  when  a  gas  contains  easily 
oxidized  constituents.  During  the  absorption  the 
phosphorus  is  always  surrounded  by  white  clouds, 
but  the  absence  of  these  must  not  be  regarded  as 
evidence  that  oxygen  is  not  present,  unless  the  gas  is 
known  to  be  free  from  all  substances  which  interfere 
with  absorption. 

The  determination  is  best  made  with  a  Hempel  gas  burette 
and  absorption  pipette,  Figs.  59a  and  59b.     The  cylindrical  bulb 


FIG.  59a 


396 


QUANTITATIVE  EXERCISES 


FIG.  59b 


of  the  pipette  is  filled  with  slender  sticks  of  phosphorus.     These 

are  prepared  by  melting  phosphorus  under  water  and  drawing  the 
liquefied  material  into  a  narrow  glass  tube, 
which  is  then  dipped  in  cold  water.  On  solid- 
ifying, the  phosphorus  contracts  and  slips  out 
of  the  tube.  The  pipette  is  filled  with  water 
until  the  phosphorus  is  completely  submerged 
and  the  liquid  rises  in  the  ascending  limb  of 
the  small  tube  at  the  side.  It  is  well  to  protect 
the  phosphorus  from  the  light  by  covering  the 
bulb  containing  it  with  blackened  paper. 
The  pipette  and  burette  are  joined  by  means 

of  a  short  piece  of  glass  tubing  of  very  small  caliber  which  is 

bent  to  two  right  angles,  and  two  rubber 

connectors,  as  shown  in  Fig.  60,  in  which 

a  so-called  "simple"  pipette  for  liquid 

reagents  only  is  exhibited.     The  manip- 
ulation is  as  follows:  The  open  end  of 

the  rubber  tube  upon  the  burette,  i.e.  the 

portion  above  the  pinchcock,  is  filled  with 

water,  and  the  glass  connecting  tube  is 

inserted  and  crowded  down  as  far  as  the 

pinchcock  will  permit.    The  glass  tube  fills 

with  water.     The  rubber  tube  upon  the 

pipette  is  then  pinched  between  the  thumb 

and  forefinger  to  expel  the  air,  and  the 

other  end  of  the  glass  tube  is  inserted. 
Fill  the  burette  with  water  and  then 

with  air  by  raising  and  afterwards  lower- 
ing the  reservoir.  .  See  that  the  water  in 

the  graduated  and  ungraduated  tubes  has 

the  same  level,  and  then  close  the  burette. 

Connect  the  burette  and  pipette  as  directed 

above  and  slowly  force  the  air  from  the  former  into  the  latter. 

After  a  few  minutes  return  the  gas  to  the  burette  and  measure 


FIG.  60 


GAS  ANALYSIS  397 

it.  Return  it  to  the  pipette  and  then  to  the  burette,  and  meas- 
ure again.  If  the  readings  are  identical,  the  absorption  is  com- 
plete. If  they  are  not,  the  gas  must  be  again  exposed  to  the 
phosphorus.  As  the  burettes  are  graduated  to  hold  100  cc., 
the  contraction,  as  read  upon  the  tube,  will  equal  the  percent- 
age by  volume  of  the  oxygen  in  the  air. 

It  is  to  be  noted  in  connection  with  the  use  of  the  Hempel 
gas-analytical  apparatus  that  gases  passing  rapidly  through 
narrow  tubes  into  those  of  larger  caliber  are  cooled  to  some 
extent  and  may  require  a  little  time  to  recover  their  former 
temperature. 

While  a  gas  is  confined  in  a  burette  care  should  be  taken  to 
have  the  reservoir  so  adjusted  that  the  pressure  upon  the  gas  is 
about  equal  to  that  of  the  air.  This  precaution  will  reduce  to  a 
minimum  the  errors  due  to  leakage  and  diffusion. 

If  a  gas  is  very  rich  in  oxygen,  the  heat  generated  by  the 
absorption  may  cause  the  phosphorus  to  melt  or  even  to  burst 
into  flames.  In  such  cases  the  proportion  of  the  oxygen  should 
be  gradually  reduced  by  transferring  only  a  part  of  the  gas  to 
the  pipette  and  then  immediately  withdrawing  it.  After  a  few 
repetitions  of  this  manipulation 
it  is  safe  to  leave  the  whole 
body  of  the  gas  in  contact  with 
the  phosphorus,  however  rich  in 
oxygen  it  may  have  been  in  the 
beginning. 


III.  BY  ABSORPTION  WITH  POTAS- 
SIUM PYROGALLATE 

There  are  required: 

1.  A   so-called    "  double    ab- 
sorption pipette,"  Fig.  61. 

2.  An  alkaline  solution  of  pyrogallol.     This  is  made  by  mix- 
ing in  the  pipette  5  grams  of  pyrogallol  dissolved  in  15  cc.  of 


398  QUANTITATIVE  EXERCISES 

water  and  120  grams  of  potassium  hydroxide  dissolved  in  80  cc. 
of  water.  Potassium  hydroxide  purified  with  alcohol  must  not 
be  used.  A  solution  of  the  concentration  given  above  liberates 
no  carbon  monoxide  during  the  absorption  of  oxygen.  At  tem- 
peratures above  15°  the  absorption  is  moderately  rapid,  while  at 
lower  temperatures  it  is  slow. 

Having  transferred  the  air  from  the  burette  to  the  pipette, 
close  the  rubber  tube  upon  the  latter  with  a  Mohr  pinchcock, 
detach  the  pipette,  and  shake  it  for  three  minutes.  Return  the 
gas  to  the  burette  and  measure.  Repeat  the  operation  until 
the  volume  of  the  residual  gas  remains  constant. 

Other  reagents  which  are  used  for  the  determination  of  oxy- 
gen in  gas  mixtures  are  chromous  chloride,  and  metallic  copper 
in  a  solution  of  ammonia  and  ammonium  carbonate. 


EXERCISE  XLIV 

DETERMINATION   OF   HYDROGEN 
I.    BY  EXPLOSION  WITH  OXYGEN 

Introduce  hydrogen,  obtained  by  the  method  of  Bunsen,  into 
the  eudiometer,  add  four  times  the  quantity  of  air  (free  from 
carbon  dioxide),  and  explode.     In  this  instance 
K  will  equal  5. 

When  hydrogen  is  to  be  determined  in  a  case 
in  which  air  cannot  be  used  owing  to  the  pres- 
ence of  nitrogen  in  it,  the  oxygen  required  for 
the  explosion  is  obtained  from  potassium  chlo- 
rate. A  minute  retort  with  a  capacity  of  from 
6  to  10  cc.,  Fig.  62,  is  blown  from  a  piece  of 
tubing  and  half  filled  with  the  dried  and  pul- 
verized salt.  After  filling,  the  end  of  the  outlet  is  bent  upwards 
to  facilitate  the  entrance  of  the  oxygen  into  the  eudiometer. 
Before  introducing  the  gas,  the  air  is  expelled  from  the  retort 
by  heating  until  the  evolution  of  oxygen  becomes  quite  rapid, 


GAS  ANALYSIS  399 

and  care  is  taken  that  the  quantity  introduced  shall  not  be  more 
than  four  times  that  of  the  hydrogen  to  be  determined. 

In  the  determination  of  hydrogen  in  the  presence  of  nitrogen, 
as  in  that  of  oxygen,  the  proportion  of  explosive  gases  to  non- 
explosive  must  not  exceed  the  ratio  of  64  to  100. 

II.   BY  ABSORPTION  WITH  PALLADIUM 

When  hydrogen  is  passed  over  palladium  sponge  which  is 
covered  with  or  contains  some  of  the  oxide  of  the  metal,  the 
oxide  is  reduced,  and  the  heat  generated  by  the  reaction  raises 
the  temperature  of  the  metal  to  the  point  at  which  it  absorbs 
the  remainder  of  the  hydrogen  with  avidity.  In  this  way  hydro- 
gen may  be  separated  from  nitrogen,  carbon  dioxide,  and  meth- 
ane. According  to  Hempel,  who  developed  the  method,  tfce 
absorption  is  not  interfered  with  by  water 
vapor  or  traces  of  ammonia,  but  may  be  pre- 
vented by  carbon  monoxide,  large  quantities 
of  the  vapors  of  benzene  and  alcohol,  and  by 
traces  of  hydrochloric  acid.  The  prevention 
of  the  occlusion  by  the  last-named  substances  he  supposes  to  be 
due  to  the  fact  that  they  appropriate  the  oxygen  of  the  palla- 
dium oxide,  but  in  burning  do  not  generate  enough  heat  to 
raise  the  metallic  palladium  to  the  temperature  at  which  it  can 
absorb  the  hydrogen. 

If  air  is  passed  over  palladium  which  has  been  employed  to 
absorb  hydrogen,  the  occluded  gas  is  burned  to  water  and  a 
small  portion  of  the  metal  is  at  the  same  time  converted  into 
oxide,  giving  a  mixture  which  is  ready  for  a  second  absorption. 

The  palladium  tube  which  is  usually  employed  in  determin- 
ing hydrogen  is  shown  in  Fig.  63.  The  parts  a  and  b  are  nearly 
capillary,  while  c  has  an  internal  diameter  of  about  5  mm.  and 
contains  from  2.5  to  3  grams  of  spongy  metal  mixed  with  some 
oxide.  The  tube  contains  air,  and  its  volume  must  be  ascer- 
tained before  the  apparatus  can  be  used  for  an  absorption, 


400  QUANTITATIVE  EXERCISES 

since,  in  the  presence  of  palladium,  hydrogen  and  oxygen  are 
converted  into  water  and  the  observed  shrinkage  in  volume  will 
be  due  not  alone  to  the  removal  of  the  former  gas,  but  in  part 
to  the  disappearance  of  the  oxygen  in  the  air  which  fills  the 
palladium  tube.  To  determine  the  volume  of  the  air  in  the 
tube,  Hempel  recommends  the  following  procedure :  The  appa- 
ratus is  closed  at  one  end  with  a  short  rubber  tube  and  a  glass 
plug,  and  connected  at  the  other  end  with  a  gas  burette  partly 
filled  with  air.  The  tube  is  then  cooled  to  9°  by  immersing  it 
in  water  of  that  temperature,  and  the  volume  of  the  air  in  the 
burette  is  read  at  atmospheric  pressure.  Afterwards  it  is  placed 
in  water  heated  to  the  boiling  point.  The  expansion  of  the  gas 
in  the  burette  which  follows  the  raising  of  the  temperature  of 
the  air  in  the  palladium  tube  from  9°  to  100°  is  equal  to  one- 
third  the  capacity  of  the  tube.  It  is  somewhat  more  accurate, 
and  at  the  same  time  more  expeditious,  to  fill  a  burette  with 
air,  to  remove  the  oxygen  with  pyrogallol,  and  then  to  insert 
the  palladium  tube  between  the  burette  and  the  pyrogallol 
pipette,  and  actually  determine  the  oxygen  in  it.  Or  one  may 
insert  the  palladium  tube  between  a  pyrogallol  pipette  and  a 
burette  filled  with  air,  and  determine  the  oxygen.  The  differ- 
ence between  the  whole  contraction  which  takes  place  and  20.9 
per  cent  of  the  volume  of  the  air  in  the  burette  is  the  volume 
of  the  oxygen  in  the  palladium  tube.  If  either  of  the  last  two 
methods  is  adopted,  the  gas  must,  of  course,  be  passed  back  and 
forth  until  the  absorption  of  the  oxygen  is  surely  complete. 
The  same  methods  may  be  employed  to  ascertain  the  capacity 
of  many  small  vessels,  or  the  extent  of  the  vacant  space  in  them 
when  they  are  partly  filled. 

The  determination  of  hydrogen  by  absorption  is  made  in  the 
following  manner:  Having  removed  other  absorbable  gases  by 
means  of  the  appropriate  reagents,  the  burette,  which  may  con- 
tain nitrogen  and  marsh  gas  in  addition  to  hydrogen,  is  attached 
to  the  palladium  tube,  and  the  latter  is  connected  in  turn  with 
a  simple  Hempel  pipette  filled  with  water.  The  portion  of  the 


GAS  ANALYSIS  401 

tube  containing  the  mixture  of  metal  and  oxide  is  immersed  in 
water  having  a  temperature  of  from  90°  to  100°,  and  the  gas  is 
passed  three  times  back  and  forth  over  the  palladium.  Finally, 
the  hot  water  is  replaced  by  cold,  and  the  residual  gas  —  to  cool 
it  —  is  again  passed  two  or  three  times  over  the  metal. 

If  a  mixture  of  hydrogen  and  marsh  gas  with  a  considerable 
excess  of  air  is  slowly  passed  over  palladium  whose  temperature 
is  kept  below  200°  by  surrounding  the  tube  containing  it  with 
water,  the  hydrogen  alone  is  burned. 


EXERCISE  XLV 
ANALYSIS   OF   ILLUMINATING   GAS 

(Method  of  Hempel) 

The  various  constituents  of  the  gas  are  determined  in  the 
following  manner  and  order: 

1.  The    so-called  hydrocarbon  vapors   (principally  benzene) 
are  absorbed  by  alcohol. 

2.  The  carbon  dioxide  is  absorbed  by  potassium  hydroxide. 

3.  The  heavy  (unsaturated)  hydrocarbons  —  ethylene,  propy- 
lene,  etc.  —  are  removed  by  fuming  sulphuric  acid. 

4.  The  oxygen,  if  present,  is  absorbed  by  phosphorus. 

5.  The  carbon  monoxide  is  absorbed  by  an  ammoniacal  solu- 
tion of  cuprous  chloride. 

6.  A  measured  portion  of  the  residual  gas,  which  consists  of 
hydrogen,  methane,  and  nitrogen,  is  mixed  with  air  and  exploded. 
The  contraction  which  follows  is  due  to  the  reactions : 

a.  2  H2  +  02  =  2  H20. 

b.  CH4  +  2  02  =  2  H20  +  C02. 

The  carbon  dioxide  is  absorbed  by  potassium  hydroxide.  Its 
volume  is  equal  to  that  of  the  methane.  The  volume  of  the 
oxygen  which  disappears  in  burning  the  methane  is  twice  that 
of  the  carbon  dioxide  or  that  of  the  methane ;  hence,  if  twice 


402 


QUANTITATIVE  EXERCISES 


the  volume  of  the  carbon  dioxide  is  subtracted  from  the  con- 
traction which  follows  the  explosion,  the  difference  is  the  con- 
traction due  to  reaction  #,  and  two-thirds  of  this  is  the  volume 
of  the  hydrogen.  The  gas  remaining  after  the  explosion  and 
the  removal  of  the  carbon  dioxide  is  nitrogen. 


The  apparatus  and  reagents  required  are : 

1.  A  Hempel  burette  filled  with  water  which  has  been  satu- 
rated with  illuminating  gas. 

2.  A  pipette  containing  1  cc.  of  absolute  alcohol,  but  other- 
wise  filled  with   mercury.     The   so-called   Hempel   explosion 

pipette,  Fig.  64,  is  employed.  The 
pipette  is  first  completely  filled  with 
mercury,  and  the  alcohol  is  run  in 
from  a  burette  at  the  open  end  of 
the  small  side  tube.  Before  using  the 
alcohol,  it  must  be  saturated  with  the 
gas.  For  this  purpose  about  50  cc. 
of  the  gas  are  drawn  in  after  the  intro- 
duction of  the  alcohol  and  the  pipette 
is  closed  and  shaken  for  three  min- 
utes, after  which  the  residual  gas  is 
expelled  and  the  pipette  closed. 

3.  An  explosion  pipette  like  2,  con- 
taining 1  cc.  of  water  over  mercury. 
The  water  is  introduced  and  saturated  with  gas  in  the  same 
manner  as  the  alcohol  in  2. 

The  alcohol  (2)  is  employed  to  remove  the  hydrocarbon 
vapors,  and  the  water  to  absorb  the  vapors  of  alcohol.  The 
second  explosion  pipette  may  be  dispensed  with ;  for  after  the 
absorption  of  the  hydrocarbon  vapors  the  alcohol  may  be  quickly 
replaced  by  an  equal  volume  of  water  which  has  previously  been 
saturated  with  the  gas. 


FIG.  64 


GAS  ANALYSIS  403 

4.  A  "  pipette  for  solid  and  liquid  reagents  "  for  the  absorp- 
tion of  carbon  dioxide,  Fig.  65.     The  cylindrical  bulb  is  tightly 
packed  with  short  rolls  of  iron  wire  gauze  and  then  filled  with 
caustic  potash  made  by  dissolv- 
ing 1  part  of  the  hydroxide  in  2 

parts  of  water. 

5.  A   pipette    of   the   form 
shown  in  Fig.  66  for  the  absorp- 
tion of  ethylene  and  other  unsat- 
urated  hydrocarbons  by  fuming 
sulphuric  acid.     The  smallest  of 
the  three  bulbs  is  filled  with  glass 
beads  by  the  maker  of  the  appa- 
ratus in  order  to  provide  a  large 
absorbing  surface  for  the  gas. 
The  function  of  the  beads  in 

this  pipette  is  the  same  as  that  of  the  wire  gauze  in  the  pipette 
for  the  absorption  of  carbon  dioxide.  All  unnecessary  exposure 
of  the  contents  to  the  air  is,  of  course,  to  be  avoided. 

6.  A  phosphorus  pipette  for  the  absorp- 
tion of  oxygen. 

7.  Two  double  pipettes,  Fig.  61,  filled 
with  an  ammoniacal  solution  of  cuprous 
chloride  for  the  absorption  of  carbon  mon- 
oxide.    The  solution  is  prepared  in  the 
following  manner:  10.3  grams  of  copper 
oxide  are  dissolved  in  from  100  to  200  cc. 
of  commercial  concentrated  hydrochloric 
acid.     The  flask  containing  the  solution 
is  filled  with  copper  wire  or  wire  gauze, 
and   allowed   to   stand   until    the  liquid 
becomes  colorless.     The  hydrochloric  acid 

solution  of  cuprous  chloride  is  poured  into  a  large  beaker  con- 
taining from  1.5  to  2  liters  of  cold  water.  When  the  precipi- 
tated cuprous  chloride  has  subsided,  the  liquid  above  is  poured 


404  QUANTITATIVE  EXERCISES 

off  as  completely  as  possible,  and  the  crystals  are  washed  with 
100  or  150  cc.  of  water  into  a  250-cc.  flask.  Ammonia  gas  is 
conducted  into  the  flask  until  the  liquid  assumes  a  pale  blue 
color,  and  no  longer.  During  the  treatment  with  ammonia  the 
flask  is  to  be  closed  with  a  doubly  perforated  stopper,  through 
which  pass  the  ammonia  delivery  tube  and  a  bent  glass  tube  which 
dips  into  a  little  mercury.  For  the  production  of  the  ammonia 
gas  about  200  cc.  of  ammonia  solution  of  0.9  specific  gravity 
will  be  required.  The  ammoniacal  solution  of  cuprous  chloride 
is  diluted  with  water  to  200  cc.  This  quantity  will  suffice  for 
one  pipette. 

Fill  the  burette  with  water  through  which  illuminating  gas 
has  been  passed  for  some  time,  and  then  with  the  gas.  Before 
collecting  the  gas  for  analysis,  however,  it  should  be  allowed  to 
waste  until  it  is  certain  that  its  composition  is  no  longer  affected 
by  the  friction  in  the  pipes. 

Connect  the  burette  with  the  alcohol  pipette  (2),  transfer  the 
gas,  disconnect,  and  shake  the  pipette  for  three  minutes.  Return 
the  gas  to  the  burette,  transfer  it  to  the  water  pipette  (3),  discon- 
nect, and  shake  for  three  minutes.  Return  the  gas  to  the  burette 
and,  after  waiting  three  minutes  for  the  water  to  run  down  the 
inner  wall  of  the  burette,  measure  the  volume.  The  contraction, 
if  100  cc.  of  the  gas  were  taken,  is  the  percentage  volume  of  the 
hydrocarbon  vapprs. 

Pass  the  gas  into  the  caustic  potash  pipette  (4)  and  withdraw 
it  immediately.  Read  after  three  minutes.  The  contraction  is 
equal  to  the  volume  of  the  carbon  dioxide. 

Connect  the  burette  with  the  fuming  sulphuric  acid  pipette 
(5),  using  a  dry  glass  tube  and  dry  rubber  for  the  purpose,  and 
taking  care  not  to  allow  the  acid  to  come  in  contact  with  the 
latter.  After  absorption,  which  requires  only  a  short  exposure, 
pass  the  gas  into  the  caustic  potash  pipette  (4)  to  remove  sul- 
phur dioxide,  and  withdraw  it  immediately.  Measure  after  three 
minutes.  The  contraction  gives  the  volume  of  the  unsaturated 
hydrocarbons. 


GAS  ANALYSIS  405 

Pass  the  gas  into  the  phosphorus  pipette  and  allow  it  to 
remain  there  three  minutes.  Withdraw  and  read  after  three 
minutes. 

Pass  the  gas  into  one  of  the  cuprous  chloride  pipettes  (7). 
Disconnect,  and  shake  the  pipette  for  two  minutes.  Withdraw 
the  gas  and  transfer  it  to  the  second  cuprous  chloride  pipette 
and  shake  for  three  minutes.  Return  the  gas  to  the  burette  and 
measure  after  three  minutes.  If  the  solution  of  cuprous  chloride 
is  perfectly  fresh,  the  treatment  of  the  gas  in  the  second  pipette 
may  be  omitted.  A  solution  in  which  a  considerable  quantity 
of  carbon  monoxide  has  been  absorbed  will  yield  up  some  of  that 
gas  whenever  another  gas  in  which  the  partial  pressure  of  carbon 
monoxide  is  small  is  brought  in  contact  with  it ;  hence  the  prac- 
tice of  using  two  pipettes,  one  of  which  always  contains  a  com- 
paratively fresh  solution  of  cuprous  chloride. 

Return  the  gas  to  the  cuprous  chloride  pipette  last  used, 
disconnect  the  burette  and  cleanse,  and  fill  it  with  water  not 
saturated  with  the  gas.  Measure  into  the  burette  from  12  to 
15  cc.  of  the  residual  gas  and  then  nearly  fill  it  with  air. 
Transfer  the  mixture  to  the  explosion  pipette  (2),  taking  care 
to  fill  the  capillary  of  the  pipette  with  water.  Close  with  a 
strong  pinchcock  and  insert  a  glass  plug  in  the  open  end  of  the 
rubber  connector.  Shake  the  pipette  vigorously,  close  the  glass 
stopcock,  and  explode.  Open  the  stopcock  and  return  the  gas 
at  once  to  the  burette.  Measure  and  transfer  to  the  caustic 
potash  pipette,  returning  the  gas  immediately  to  the  burette. 
The  method  of  calculating  the  quantities  of  methane  and  hydro- 
gen has  already  been  explained.  The  nitrogen  is  found  by 
difference.  It  is  well  to  pass  the  gas,  after  removing  the  carbon 
dioxide  and  measuring,  into  the  phosphorus  pipette  in  order  to 
demonstrate  the  sufficiency  of  the  supply  of  oxygen. 

If  preferred,  the  hydrogen  may,  of  course,  be  determined  sepa- 
rately by  absorbing  it  in  palladium. 

Some  unfortunate  accidents  have  resulted  from  the  use  of 
the  explosion  pipette,  and  the  student  is  warned  to  insure  his 


406  QUANTITATIVE  EXERCISES 

safety  by  covering  the   apparatus   with  a  strong   box    before 
closing  the  circuit. 

For  further  information  regarding  the  quantitative  determi- 
nation of  gases,  the  student  is  referred  to  the  works  of  Bunsen, 
Winkler,  and  Hempel. 


OF    THE 

UNIVERSITY 

OF 


CHAPTER  XVIII 

THE  METALS  OF  THE  ALKALIES  AND  OF  THE 
ALKALINE  EARTHS 


EXERCISE  XLVI 

DETERMINATION  OF  POTASSIUM  AS  DOUBLE 
POTASSIUM-PLATINUM  CHLORIDE 

Weigh  about  0.1  gram  of  pure  potassium  chloride  into  a 
small  porcelain  dish.  Dissolve  the  salt  in  a  little  water  and  add 
somewhat  more  of  a  nearly  neutral  solution  of  .platinum  chloride 
than  is  necessary  to  precipitate  all  of  the  potassium  as  the 
double  compound.  Evaporate  upon  the  water  bath  to  a  sirupy 
consistency.  Allow  the  dish  to  cool,  and  add  to  the  contents 
10  or  15  cc.  of  strong  alcohol.  After  mixing,  allow  the  con- 
tents of  the  dish  to  remain  undisturbed  for  an  hour  or  more 
and  then  filter. 

If  a  Gooch  filter  is  available,  this  should  be  used.  Otherwise 
filter  through  a  very  small  paper.  Wash  with  alcohol  and  allow 
the  paper  to  dry  at  the  temperature  of  the  air.  When  dry, 
remove  the  precipitate  as  completely  as  may  be  to  a  weighed 
platinum  or  porcelain  crucible.  Replace  the  filter  in  the  funnel 
and  dissolve  the  precipitate  remaining  upon  it  in  a  little  boiling 
water  and  wash  the  paper  clean,  collecting  the  solution  and 
washings  in  a  small  beaker  or  a  porcelain  dish.  Place  the 
weighed  crucible  containing  the  greater  portion  of  the  platinum 
salt  upon  a  water  bath  and  evaporate  in  it  to  dryness  the  solu- 
tion obtained  from  the  filter  paper.  Heat  to  constant  weight  in 
an  air  bath  at  a  temperature  of  130°. 

407 


408  QUANTITATIVE  EXERCISES 

After  weighing,  place  a  small  quantity  of  pure  oxalic  acid  in  the 
crucible.  Heat,  with  the  cover  on,  gently  at  first  and  afterwards 
more  strongly.  Weigh  and  then  repeat  the  treatment  with  oxalic 
acid,  etc.  The  residue  after  such  treatment  is  a  mixture  of  metal- 
lic platinum  and  potassium  chloride,  Pt  +  2  KCl.  Dissolve  out  the 
chloride  and  weigh  the  platjnum.  The  same  decomposition  of  the 
double  salt  may  be  effected  by  heating  it  in  a  current  of  hydrogen. 

The  double  chloride  of  potassium  and  platinum  is  not  wholly 
insoluble  in  strong  alcohol.  Therefore  proceed  with  the  first 
filtrate  in  the  following  manner:  Add  to  it  a  small  quantity  of 
pure  sodium  chloride  in  solution  and  evaporate  at  a  temperature 
not  exceeding  75°  until  all  of  the  alcohol  has  been  removed. 
Add  a  little  platinum  chloride.  Evaporate  on  the  water  bath 
nearly  to  dryness.  Treat  the  residue  with  strong  alcohol,  etc. 
The  sodium  chloride  is  used  to  prevent  the  decomposition  of 
the  platinum  chloride  by  the  alcohol  during  the  evaporation. 
More  of  the  platinum  chloride  is  added,  before  the  final  treat- 
ment with  alcohol,  in  order  to  convert  any  excess  of  sodium 
chloride  into  the  double  sodium-platinum  salt,  which  is  soluble 
in  alcohol,  while  sodium  chloride  is  insoluble.  If  the  last  step 
is  omitted,  the  small  quantity  of  potassium  salt  which  is  obtained 
from  the  filtrate  may  be  mixed  with  sodium  chloride. 

When  the  salt  in  which  potassium  is  to  be  determined  is  not 
the  chloride,  also  when  other  acids  or  their  salts  are  present, 
the  procedure  is  somewhat  modified.  If  the  acids  are  volatile 
ones  —  e.g.  nitric  acid,  acetic  acid,  etc.  —  the  solution  of  the 
material  is  repeatedly  evaporated  with  hydrochloric  acid  before 
treating  with  platinum  chloride.  If  nonvolatile  acids,  like 
phosphoric  and  boric  acids,  are  present,  the  solution  is  evapo- 
rated to  a  small  volume  with  hydrochloric  acid,  and  then  treated 
with  an  excess  of  platinum  chloride  and  a  considerable  quantity 
of  strong  alcohol.  After  standing  for  24  hours,  the  precipitate 
is  filtered,  washed  with  alcohol,  etc. 

The  determination  must,  of  course,  be  conducted  in  an  atmos- 
phere which  is  free  from  ammonia  and  from  hydrogen  sulphide. 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     409 

According  to  Fresenius,  potassium-platinum  chloride  is  sol- 
uble in  12,083  parts  of  absolute  alcohol,  and  in  3775  parts  of 
76  per  cent  alcohol. 

THE  MONROE  FILTER 

A  filter  which  is  to  be  recommended  for  the  collection  of 
potassium-platinum  chloride  and  many  other  precipitates  is  that 
proposed  by  C.  E.  Monroe  (Chemical  News,  58,  101).  It  is  a 
modification  of  the  Gooch  filter,  in  which  platinum  sponge  is 
used  in  the  place  of  asbestus.  The  perforated  Gooch  crucible  is 
placed  upon  filter  or  blotting  paper  and  its  bottom  covered  to 
a  depth  of  4  or  5  mm.  with  moist  ammonium-platinum  chloride. 
The  crucible  is  cleansed,  dried,  covered,  and  placed  in  its  cap, 
and  the  double  salt  is  gradually  decomposed  by  heat.  If  cracks 
appear  in  the  filter,  they  may  be  filled  by  rubbing  down  with 
the  smooth  end  of  a  glass  rod,  or  by  introducing  more  of  the 
double  salt  and  reheating  the  crucible.  If  the  crevices  are 
large,  it  will  be  found  advantageous  to  employ  both  of  these 
means  of  closing  them. 

A  filter  of  this  kind  is  useful  when  the  substance  is  of  such 
a  character  that  a  paper  upon  which  it  is  collected  cannot  be 
burned,  and  also  for  the  filtration  of  precipitates  which  are 
soluble  in  acids.  The  oxygen  in  the  spongy  platinum  must  be 
removed  with  boiling  water  before  filtering  solutions  containing 
hydrochloric  acid.  Otherwise  platinum  chloride  will  be  found 
in  the  filtrate. 

OTHER  METHODS  OF  DETERMINING  POTASSIUM 

If  potassium  is  in  combination  with  any  volatile  acid,  and  is 
otherwise  free  from  nonvolatile  matter,  it  may  be  evaporated 
with  hydrochloric,  nitric,  or  sulphuric  acid,  and  weighed  as  chlo- 
ride, nitrate,  or  sulphate.  If  either  of  the  first  two  is  employed, 
the  evaporation  must  be  repeated  with  fresh  portions  of  the  acid 
until  the  weight  of  the  residue  is  found  to  be  constant. 


410  QUANTITATIVE  EXERCISES 

Potassium  may  also  be  precipitated  from  an  alcoholic  solution 
by  hydrofluosilicic  acid,  and  subsequently  determined  by  means 
of  a  standard  solution  of  potassium  or  sodium  hydroxide. 

The  concentrated  solution  of  the  salt  is  treated  with  an  excess 
of  hydrofluosilicic  acid  and  an  equal  volume  of  strong  alcohol. 
The  precipitate  is  collected  upon  a  filter  and  washed  with  a 
mixture  of  equal  volumes  of  alcohol  and  water  until  a  neutral 
nitrate  is  obtained.  It  is  then  returned,  together  with  the  filter, 
to  the  beaker  in  which  the  precipitation  was  made,  treated 
with  boiling  water  and  titrated  with  the  standard  alkali,  with 
use  of  litmus  as  the  indicator.  The  reaction  which  takes  place 
is  represented  by  the  following  equation : 

K2SiF6  +  4  KOH  =  6  KF  +  SiO2  +  2  H2O. 

Free  acids,  especially  sulphuric  acid,  should  be  removed  by 
evaporation  before  attempting  to  precipitate  potassium  as  the 
fluosilicate. 

EXERCISE  XLVII 

SEPARATION  AND   DETERMINATION  OF   POTASSIUM 
AND   SODIUM 

Weigh  about  1  gram  of  pure  potassium-sodium  tartrate, 
KNaC4H4O64H2O,  into  a  platinum  dish.  Heat  very  cautiously 
until  all  volatile  matter  has  been  expelled,  and  then  raise  the  tem- 
perature of  the  charred  residue  to  a  dull  red.  When  cold,  add 
water  and  heat  for  some  time  nearly  to  the  boiling  point.  Filter 
through  a  small,  "  ashless  "  paper  into  a  porcelain  vessel.  Wash 
the  dish,  the  carbon,  and  the  filter  thoroughly  with  hot  water. 
Return  the  undissolved  material,  together  with  the  paper,  to  the 
platinum  dish,  and  incinerate  at  the  lowest  practicable  tempera- 
ture. The  ash  should  consist  of  alkaline  carbonates  except  for  a 
minute,  hardly  weighable,  quantity  of  insoluble  matter  derived 
from  the  paper.  Acidify  the  solution  of  the  alkalies  with  hydro- 
chloric acid  and  evaporate  it  to  a  .small  volume  in  the  platinum 
dish.  Pour  the  liquid  into  a  weighed  platinum  crucible  and 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     411 

continue  the  evaporation  to  dryness,  adding  from  time  to  time  the 
water  used  in  washing  the  dish.  Heat  the  crucible  for  a  long 
time  very  moderately,  then  cover  it  and  gradually  raise  the  tem- 
perature to  a  dull  red.  Weigh  the  mixed  chlorides  and  dissolve 
them  in  water.  Transfer  the  solution  to  a  porcelain  dish  and  add 
a  quantity  of  platinum  chloride  which  will  certainly  more  than 
suffice  to  convert  both  potassium  and  sodium  into  the  platinum 
double  salts.  Evaporate  the  solution,  at  a  temperature  below 
the  boiling  point  of  water,  to  a  thick  liquid.  Add  strong  alcohol 
and  proceed  in  other  respects  as  directed  in  Exercise  XLVI. 

To  test  the  purity  of  the  weighed  potassium-platinum  chlo- 
ride, examine  it  with  a  magnifying  glass  and  then  treat  it 
repeatedly  with  small  -quantities  of  cold  water,  pouring  the 
solutions  thus  obtained  into  a  porcelain  dish.  Add  a  little 
more  platinum  chloride,  evaporate,  treat  with  alcohol,  etc.  The 
second  weight  of  the  double  salt  should  agree  very  closely  with 
the  first.  The  filtrate  from  the  second  precipitation  is  to  be 
added  to  the  solution  containing  the  sodium  salt. 

The  quantity  of  the  sodium  can,  of  course,  be  calculated  from 
the  weights  of  the  mixed  chlorides  and  of  the  potassium  chloride 
in  the  double  salt.  The  result  of  the  calculation  is,  however,  to 
be  confirmed  in  the  following  manner:  Evaporate  the  solution 
containing  the  sodium  salt  and  the  excess  of  the  platinum  chlo- 
ride to  dryness  in  a  porcelain  crucible.  Heat  the  residue  moder- 
ately in  a  current  of  hydrogen  and  extract  the  sodium  chloride 
with  water.  Evaporate  the  solution  in  a  weighed  platinum 
crucible,  and  proceed  as  in  the  determination  of  the  weight  of 
the  mixed  chlorides. 


SEPARATION   OF   POTASSIUM   AND   SODIUM   FROM 
OTHER   SUBSTANCES 

When  potassium  and  sodium  are  to  be  determined  they  are 
first  isolated  from  all  other  substances  in  the  form  of  chlorides  if 
practicable.  How  this  may  be  accomplished  when  the  alkalies 


412  QUANTITATIVE  EXERCISES 

are  in  combination  with  nonvolatile  organic  acids  has  been 
learned  from  the  preceding  exercise. 

The  nitrates,  bromides,  iodides,  cyanides,  acetates,  and,  in 
general,  the  compounds  of  the  alkalies  with  the  more  volatile 
acids  are  readily  converted  into  chlorides  by  repeated  evapora- 
tion to  dry  ness  with  hydrochloric  acid. 

Sulphates,  phosphates,  borates,  and  some  other  salts  require  a 
different  treatment,  and  usually  such  a  treatment  as  will  effect  a 
separation  of  the  acid,  leaving  the  alkaline  metals  in  combination 
with  chlorine,  or  in  a  condition  to  be  converted  into  chlorides. 

1.  SULPHATES 

Various  methods  are  employed  for  the  conversion  of  the  sul- 
phates of  the  alkalies  into  chlorides.  (1)  The  dry  salts  are 
heated  in  a  platinum  crucible  with  ammonium  chloride,  and 
the  process  is  repeated  until  a  constant  weight  is  obtained. 

(2)  The  solution  of  the  sulphates  is  treated  with  a  very  moder- 
ate excess  of  pure  lead  acetate  and  a  little  alcohol,*  and  filtered. 
The  filtrate  is  treated  with  hydrogen  sulphide  to  precipitate 
the  excess  of  the  lead,  and  again  filtered.    Finally,  the  solu- 
tion, which  now  contains  .the   potassium  and  sodium  in  the 
form  of  acetates,  is  repeatedly  evaporated  with  hydrochloric  acid. 

(3)  The  precipitation  of  the  sulphuric  acid  by  barium  chloride 
or   by  strontium   chloride    and   alcohol  —  a    method  formerly 
in  use  —  has  been  abandoned  owing  to  the  tendency  on  the 
part  of  barium  and  strontium  when  precipitated  as  sulphates 
to  carry  down  with  them  the  salts  of  the  alkalies. 

2.  PHOSPHATES 

Phosphoric  acid  may  be  separated  from  the  metals  of  the 
alkalies  by  precipitation  either  as  the  phosphate  of  lead  or  as 
the  phosphate  of  iron.  (1)  The  solution  is  treated  with  a  little 
ammonium  chloride  or  with  hydrochloric  acid,  and  then  with 
lead  acetate  as  long  as  a  precipitate  forms.  Afterwards  there 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     413 

is  added  a  quantity  of  lead  carbonate,  prepared  by  treating  a 
solution  of  the  acetate  with  ammonium  carbonate.  After  long 
digestion  the  precipitate  —  consisting  of  lead  phosphate,  chlo- 
ride, and  carbonate  —  is  filtered  and  washed.  The  lead  in  the 
filtrate  is  precipitated  by  hydrogen  sulphide.  The  lead  sulphide 
is  collected  upon  a  paper  and  washed,  and  the  nitrate  is  repeat- 
edly evaporated  with  hydrochloric  acid.  (2)  The  solution  is 
acidified  with  hydrochloric  acid,  treated  with  an  excess  of  ferric 
chloride,  and  then  considerably  diluted  with  water.  Ammonia 
is  added  to  neutral  reaction  and  the  solution  i's  boiled.  The 
precipitate,  which  contains  all  of  the  iron  and  the  phosphoric 
acid,  is  filtered,  and  the  filtrate,  which  contains  the  potassium 
and  sodium,  is,  as  usual,  evaporated  with  hydrochloric  acid. 

3.  BORATES 

Boric  acid  is  separated  from  the  alkaline  metals,  and  also 
from  all  other  metals,  as  boron  fluoride.  The  material  is 
digested  for  some  time  in  a  platinum  vessel  with  aqueous 
hydrofluoric  acid,  and  then  cautiously  treated  with  sulphuric 
acid.  The  vessel  is  afterwards  heated  —  moderately  at  first, 
but  finally  to  a  temperature  sufficient  for  the  expulsion  of  the 
excess  of  sulphuric  acid.  The  boron  is  completely  volatilized, 
while  the  alkalies  remain  as  sulphates.  Boric  acid  may  also 
be  separated  from  the  alkaline  and  from  other  metals  by  con- 
verting it  into  a  volatile  ethereal  salt  by  treating  the  borate  with 
sulphuric  acid  and  methyl  or  ethyl  alcohol. 

EXERCISE   XLVIII 

DETERMINATION  OF  POTASSIUM  AND  SODIUM 
IN  A  SILICATE 

Weigh  into  a  platinum  dish  from  0.7  to  1.0  gram  of  a  soda- 
potash  glass  —  a  fragment  of  a  Florence  flask  will  answer  as 
material — which  has  been  finely  pulverized  and  dried  at  100° 


414  QUANTITATIVE  EXERCISES 

or  at  ordinary  temperatures  in  a  desiccator.  Treat  the  powder, 
little  by  little,  with  a  fuming  aqueous  solution  of  hydrofluoric 
acid  which  has  been  purified  by  distillation  from  a  platinum 
retort.  Stir  the  mixture  occasionally  with  a  platinum  wire  or 
spatula.  Warm  the  dish  for  some  time  upon  a  water  bath  and 
then  add,  drop  by  drop,  sulphuric  acid  which  has  been  diluted 
with  an  equal  volume  of  water  until  more  than  enough  has  been 
introduced  to  convert  all  of  the  bases  into  sulphates.  Evapo- 
rate for  a  time  upon  the  water  bath  and  then  gradually  heat  to 
higher  temperatures  until  nearly  all  of  the  excess  of  sulphuric 
acid  has  been  expelled.  Allow  the  dish  to  cool,  add  concen- 
trated hydrochloric  acid,  and  then  set  it  aside  for  several  hours. 
Dilute  with  water  and  warm  gently.  The  material  in  the  dish 
should  dissolve  completely.  If  it  does  not,  filter  through  an 
ashless  paper,  wash,  and  burn  the  filter,  and  treat  the  residue 
in  the  same  manner  as  the  original  material. 

The  solution  will  contain  potassium,  sodium,  and  calcium — 
the  principal  metallic  constituents  of  the  glass  —  as  sulphates, 
also  small  quantities  of  the  sulphates  of  iron  and  aluminium, 
and  possibly  of  manganese.  Evaporate  until  the  greater  por- 
tion of  the  hydrochloric  acid  has  been  expelled.  Transfer  the 
contents  of  the  dish  to  a  beaker,  washing  the  former  well  with 
water.  Heat  nearly  to  the  boiling  point  and  add  a  moderate 
excess  of  ammonium  oxalate  and  enough  ammonia  "to  render  the 
liquid  distinctly  alkaline.  Set  the  beaker  in  a  warm  place  and 
allow  it  to  stand  undisturbed  for  24  hours.  Decant  the  clear 
liquid  through  a  small  paper,  and  wash  the  precipitate  in  the 
beaker  with  several  small  portions  of  hot  water  before  allowing 
it  to  come  upon  the  paper.  Wash  well  the  beaker,  and  filter 
without  attempting  to  bring  all  of  the  oxalate  in  the  former 
into  the  latter.  To  the  filtrate  add  a  few  drops  of  ammonium 
sulphide.  If  a  precipitate  appears,  add  a  little  more  of  the 
reagent  and  set  aside  for  several  hours.  Filter  and  wash 
with  water  containing  a  small  quantity  of  ammonium  sulphide. 
Acidify  the  filtrate  slightly  with  sulphuric  acid  and  evaporate  to 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     415 

dry  ness,  first  in  porcelain  and  then  in  a  large  weighed  platinum 
crucible.  Heat  the  residue  cautiously  with  the  cover  on  the 
crucible  as  long  as  ammonium  salts  are  observed  to  escape. 
Allow  the  crucible  to  cool,  add  dry  ammonium  carbonate,  and 
again  heat.  Repeat  the  treatment  with  ammonium  carbonate 
and  the  subsequent  heating  two  or  three  times  and  then  weigh. 
Dissolve  the  mixture  of  sulphates  in  a  little  water;  the  solution 
should  be  clear.  If  it  is  not  clear,  the  treatment  with  ammonium 
oxalate  must  be  repeated,  but  not  that  with  ammonium  sulphide. 
Add  a  little  ammonium  sulphate  to  the  solution,  evaporate,  and 
ignite,  using  ammonium  carbonate  as  before  to  assist  in  the 
removal  of  the  acid  in  the  acid  sulphates. 

Having  found  the  weight  of  the  mixture  of  potassium  and 
sodium  sulphates,  add  dry  ammonium  chloride  which  contains 
no  nonvolatile  matter,  and  gently  ignite.  Repeat  the  addition 
of  ammonium  chloride  and  the  ignition  until  a  constant  weight 
is  obtained. 

Having  found  the  weight  of  the  chlorides,  separate  and  deter- 
mine the  potassium  and  sodium  as  directed  in  Exercise  XLVII. 

The  calcium  in  the  glass  should  also  be  determined  by  heat- 
ing the  oxalate  and  weighing  the  resulting  carbonate. 

DETERMINATION  OF    AN  ALKALINE  CARBONATE  IN  A 
CAUSTIC  ALKALI 

Most  caustic  alkalies  contain  more  or  less  of  an  alkaline  car- 
bonate, owing  either  to  the  incomplete  conversion  of  the  latter 
in  the  process  of  manufacturing  of  the  former,  or  to  the  expos- 
ure of  the  product  to  the  carbonic  acid  of  the  air.  There  are 
various  methods  of  determining  the  amount  of  the  carbonate  in 
such  materials.  Of  these,  the  following  is  the  simplest :  A 
weighed  quantity  of  the  alkali  is  dissolved  in  water  and  diluted 
to  a  known  volume.  Measured  portions  of  the  solution  are  then 
withdrawn  and  titrated  .with  a  standard  acid,  using  methyl 
orange  as  the  indicator.  In  this  way  the  sum  of  the  caustic 


416  QUANTITATIVE  EXERCISES 

alkali  and  the  carbonate  is  found.  Another  weighed  portion  of 
the  material  is  dissolved  in  water,  treated  with  enough  barium 
chloride  to  precipitate  the  carbonic  acid,  and  then  diluted  to  a 
known  volume.  The  measuring  flask  in  which  the  precipitation 
was  made  is  closed,  shaken,  and  then  set  aside  until  the  barium 
carbonate  settles.  Measured  portions  of  the  clear  solution  are 
neutralized  with  the  standard  acid,  using  either  phenolphthalein 
or  litmus  as  the  indicator.  By  the  last  operation  the  quantity 
of  the  caustic  alkali  is  determined,  and  the  difference  between 
this  and  the  total  alkali  —  previously  estimated  —  is  the  amount 
of  the  carbonate.  If  great  accuracy  is  required,  a  correction 
must  be  made  for  the  space  occupied  by  the  barium  carbonate. 
Its  volume  may,  of  course,  be  found  by  dividing  its  estimated 
weight  by  its  specific  gravity. 

The  alkaline  hydroxides  and  carbonates  may  be  estimated  in 
the  same  solution  in  the  following  manner  :  The  liquid  is  treated 
with  phenolphthalein  and  then  titrated  to  neutral  reaction  with 
a  standard  acid.  Methyl  orange  is  then  added  and  the  titration 
continued  to  acid  reaction.  The  equations  given  below  repre- 
sent the  reactions  which  take  place  : 

1.  When  acid  is  added  until  the  phenolphthalein  loses  its 

color  : 

KOH  +  HC1  =  KC1  +  H20, 

K2CO3  +  HC1  =  KC1  +  HKCO3. 

2.  When  acid  is  added  until  the  methyl  orange  reddens  : 

HKC03  +  HC1  =  KC1  +  H20  +  CO2. 

The  alkali  in  the  form  of  carbonate  is  equivalent  to  twice  the 
acid  used  in  the  last  titration. 


DETERMINATION  OF  ACID  CARBONATES  IN  THE 
PRESENCE  OF  NEUTRAL  CARBONATES 

The  solution  of  the  material  is  treated  in  a  measuring  flask 
with  a  measured  but  excessive  quantity  of  a  dilute  standard 
ammonia  and  then  with  an  excess  of  barium  chloride.  The 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     417 

flask  is  filled  to  the  mark  with  boiled  water  and  the  barium 
carbonate  allowed  to  subside.  The  excess  of  the  ammonia  is 
ascertained  by  titrating  measured  portions  of  the  clear  solution 
with  a  standard  acid.  The  ammonia  added,  but  not  found  in 
the  subsequent  titration,  has  disappeared  in  the  reaction, 

HKC03  +  NH3  =  NH4KC03, 

and  each  molecule  of  the  missing  ammonia  represents  one  mole- 
cule of  acid  carbonate. 

If  a  standard  solution  of  sodium  or  potassium  hydroxide,  free 
from  carbonate,  is  used  instead  of  ammonia,  and  phenolphthalein 
is  employed  as  the  indicator,  the  titration  of  the  excess  of  the 
alkali  may  be  made  in  the  presence  of  the  barium  carbonate,  and 
the  trouble  of  diluting  to  a  known  volume  avoided. 

In  the  analysis  of  the  cruder  commercial  products,  the  prob- 
able presence  of  alkaline  sulphides,  sulphites,  aluminates,  and 
silicates  must  be  taken  into  account. 


EXERCISE  XLIX 

SEPARATION  AND  DETERMINATION  OF  BARIUM, 
STRONTIUM,  AND  CALCIUM 

(By  the  methods  recommended  by  Fresenius) 

In  addition  to  the  more  ordinary  reagents,  there  are  required : 
1.  Ammonium  chromate.  To  prepare  this,  dissolve  a  weighed 
quantity  of  crystallized  chromic  anhydride  in  water  and  treat 
with  ammonia  until  the  solution  acquires  the  characteristic  yellow 
color  of  the  neutral  chromates  of  the  alkalies.  Add  a  second 
equal  portion  of  chromic  anhydride,  and  evaporate  the  solution 
to  the  point  of  crystallization.  Recrystallize  the  product  until 
the  salt  gives  no  reaction  for  sulphuric  acid  when  reduced  by 
alcohol  and  hydrochloric  acid  and  treated  with  barium  chloride. 
Dissolve  the  pure  salt  in  water  and  add  ammonia  until  the  solu- 
tion, when  diluted,  reacts  neither  acid  nor  alkaline. 


418  QUANTITATIVE  EXERCISES 

2.  Anhydrous  ether.     Heat  the  commercial  product  upon  a 
water  bath  with  thin  shavings  of  clean  sodium  until  the  evolution 
of  hydrogen  ceases,  and  then  distill.     The  flask  containing  the 
ether  must  be  attached  to  a  return  condenser;  and  as  it  is  unsafe 
to  have  a  flame  in  the  vicinity,  the  water  employed  in  the  bath 
should  be  heated  elsewhere  and  introduced  as  occasion  requires. 

3.  Absolute  alcohol.     This  may  be  prepared  in  either  of  two 
ways.     First,  95  per  cent  alcohol  is  treated  with  250  grams  per 
liter  of  calcium  oxide  and  heated  upon  a  water  bath  in  a  flask 
to  which  an  inverted  condenser  is  attached.     After  an  hour  or 
more,  the  alcohol  is  distilled.     If  it  contained  in  the  beginning 
more  than  5  per  cent  of  water,  the  treatment  with  lime  must  be 
repeated.    Or,  second,  the  alcohol  is  allowed  to  remain  for  a 
long  time  in  contact  with  anhydrous   copper  sulphate  and  is 
then  distilled.     The  absorption  of  the  water  by  the  sulphate  is, 
of  course,  greatly  facilitated  by  agitation. 

4.  Pure    barium,   strontium,   and  calcium   carbonates.     The 
first  two  are  obtained  by  treating  solutions  of  the  pure  chlorides 
with  ammonia  and  ammonium  carbonate,  and  filtering,  washing, 
and  igniting  the  precipitate  in  a  platinum  crucible.     Calcium 
carbonate  may  be  prepared  in  the  same  manner,  or  Iceland  spar 
may  be  used. 

Weigh  into  a  beaker  about  0.3  gram  of  each  of  the  three 
carbonates.  Dissolve  with  the  least  practicable  quantity  of 
hydrochloric  acid  and  evaporate  the  solution  to  dryness  to 
remove  the  excess  of  the  acid.  Dissolve  the  chlorides  in  water 
and  dilute  to  300  cc.  Add  half  a  dozen  drops  of  acetic  acid 
(sp.  gr.  1.065),  and  heat  the  solution  nearly  to  the  boiling  point. 
Add  12  or  15  cc.  of  a  10  per  cent  ammonium  chromate  solution 
and  set  aside  for  an  hour  or  longer.  Decant  the  liquid  through 
a  filter.  Wash  the  precipitate  with  water  strongly  colored  with 
ammonium  chromate  until  the  filtrate  gives  no  precipitate  with 
ammonia  and  ammonium  carbonate,  and  then  with  pure  warm 
water  until  the  filtrate  gives  only  a  slight  red  color  with  sil- 
ver nitrate.  The  precipitate  contains  all  of  the  barium,  some 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     419 

strontium,  and  a  little  calcium.  Return  it,  as  completely  as  may 
be,  with  the  aid  of  the  wash  bottle,  to  the  beaker,  and  dissolve 
what  remains  on  the  filter  with  a  little  warm  and  very  dilute 
nitric  acid.  Wash  the  filter  into  the  beaker  and  dissolve  the 
remainder  of  the  precipitate  in  the  smallest  possible  quantity  of 
warm  dilute  nitric  acid.  Dilute  the  solution  to  200  cc.  and  heat. 
Add  slowly  5  cc.  of  ammonium  acetate  (30  per  cent  solution)  to 
neutralize  the  excess  of  the  nitric  acid,  and  then  ammonium  chro- 
mate  until  the  odor  of  acetic  acid  disappears.  At  the  end  of  an 
hour  decant  the  liquid  through  a  filter.  Digest  the  precipitate 
remaining  in  the  beaker  with  hot  water.  Let  it  cool  and  again 
decant.  Bring  the  precipitate  upon  the  filter  with  cold  water 
and  wash  it  with  the  same  until  the  filtrate  gives  only  a  slight 
color  reaction  with  silver  nitrate.  The  precipitate  now  contains 
all  of  the  barium  (except  slight  traces),  and  is  free  from  stron- 
tium and  calcium.  The  last  filtrate,  which  contains  the  strontium 
and  calcium  retained  by  the  barium  chromate  first  precipitated, 
is  to  be  added  to  the  first. 

Dry  the  paper  on  which  the  barium  chromate  is  collected,  and 
remove  the  precipitate  to  a  weighed  platinum  crucible.  Fold 
the  paper  loosely,  place  it  in  the  crucible,  and  incinerate  at  a 
low  temperature.  Continue  gently  to  ignite  until  the  chromate 
reduced  during  the  burning  of  the  paper  has  been  reoxidized. 

Concentrate  by  evaporation  the  filtrate  which  contains  the 
strontium  and  calcium,  adding  a  little  nitric  acid.  Treat  the 
solution  with  ammonia  and  a  moderate  excess  of  ammonium 
carbonate  and  allow  it  to  stand  for  some  hours  in  a  warm  place. 
Filter  and  wash  with  water.  Dissolve  the  precipitate  of  stron- 
tium and  calcium  carbonates  in  nitric  acid  and  evaporate  the 
solution  in  a  porcelain  dish.  Heat  the  residue  in  an  air  bath 
for  a  long  time  at  a  temperature  of  130°.  Grind  the  dried 
nitrates  to  a  fine  powder  with  a  porcelain  pestle.  Add  5  cc. 
of  a  mixture  of  equal  volumes  of  ether  and  alcohol.  Grind 
the  materials  together  quickly  and  pour  the  liquid  portion,  as 
perfectly  as  may  be,  into  a  small  flask.  Repeat  the  extraction  five 


420  QUANTITATIVE  EXERCISES 

times.  Evaporate  the  ether  and  alcohol  adhering  to  the  residue, 
and  dry  it  again  at  130°.  Transfer  it  to  the  flask  containing 
the  solution  and  wash  the  dish  with  the  mixture  of  ether  and 
alcohol.  Close  the  flask  and  agitate  the  contents  from  time  to 
time  for  24  hours.  Filter  through  a  small  dry  paper  and  wash 
well  with  the  ether-alcohol  mixture. 

The  filtrate  contains  the  calcium.  Add  to  it  a  slight  excess 
of  sulphuric  acid.  After  12  hours  collect  the  precipitate  upon 
a  Gooch  filter,  wash  with  alcohol,  dry,  ignite  at  a  very  low  red 
heat,  and  weigh. 

Dissolve  the  strontium  nitrate  remaining  on  the  filter  and  in  the 
flask  in  a  small  quantity  of  water.  Add  to  the  solution  an  equal 
volume  of  alcohol  and  a  moderate  excess  of  sulphuric  acid.  After 
12  hours  collect  the  precipitate  on  a  filter,  wash  with  a  mixture  of 
equal  volumes  of  water  and  alcohol,  dry,  ignite,  and  weigh. 

Instead  of  separating  the  barium  from  the  other  metals  and 
afterwards  the  calcium  from  the  strontium  as  prescribed,  the 
mixture  of  the  three  carbonates  may  be  converted  at  once  into 
nitrates  and  the  calcium  salt  extracted  from  the  dried  residue 
with  the  mixture  of  ether  and  alcohol.  The  subsequent  sepa- 
ration of  barium  from  strontium  by  means  of  ammonium  chro- 
mate  would  not  require  any  modification  in  consequence  of  the 
absence  of  calcium. 

Within  recent  years  an  elaborate  examination  of  all  the 
methods  hitherto  proposed  and  in  use  for  the  separation  of  the 
alkaline  earths  has  been  made  by  Fresenius.  As  regards 
the  separation  of  calcium  from  strontium,  only  one,  the  ether- 
alcohol  method,  was  found  to  yield  satisfactory  results.  For 
the  separation  of  barium  from  strontium  and  calcium,  two 
methods,  in  modified  forms,  were  found  to  be  available.  One 
of  these,  the  better  of  the  two,  has  been  presented  in  the  preced- 
ing exercise.  The  other,  which  is  based  on  the  comparative 
insolubility  of  barium  silicofluoride,  is  briefly  given  below. 

The  solution  containing,  besides  barium,  strontium  or  cal- 
cium, or  both  strontium  and  calcium,  as  chlorides,  is  diluted  to 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     421 

100  cc.  and  treated  with  an  excess  of  pure  hydrofluosilicic  acid. 
After  12  hours  the  precipitate  is  collected  upon  a  filter  and 
washed  with  cold  water  until  the  filtrate  gives  no  reaction  for 
chlorine.  The  material  upon  the  paper  is  free  from  strontium 
and  calcium,  but  the  filtrate  contains  a  small  fraction  of  the 
barium,  possibly  amounting  to,  but  not  exceeding,  1  part  of 
BaSiF6  to  3000  parts  of  the  liquid.  To  the  filtrate  there  is 
added  from  five  to  six  times  the  volume  of  half-normal  sulphuric 
acid  which  is  estimated  to  be  necessary  for  the  precipitation  of 
the  barium,  on  the  assumption  that  the  solution  is  saturated 
with  the  silicofluoride.  The  precipitate,  which  is  free  from  cal- 
cium but  contains  a  little  strontium,  is  collected  upon  a  filter, 
washed,  dried,  and,  after  burning  the  paper,  converted  into  car- 
bonate by  fusion  with  sodium  carbonate.  The  carbonates  are 
converted  into  chlorides  and  dissolved  in  a  small  volume  of 
water.  To  this  solution  there  is  added  a  small  quantity  of  hydro- 
fluosilicic acid,  and  30  minutes  later  a  volume  of  alcohol  equal 
to  one-third  the  total  volume  of  the  solution.  The  precipitate  is 
collected  and  washed  with  alcohol  which  has  been  diluted  with 
an  equal  volume  of  water.  The  second  precipitate  of  barium 
silicofluoride  is  free  from  strontium.  It  is  not  practicable  to 
determine  barium  by  weighing  the  silicofluoride.  It  is  therefore 
necessary  to  convert  the  metal  in  the  two  precipitates  into  sul- 
phates. To  determine  the  strontium  and  calcium  in  the  filtrates, 
they  must  be  precipitated  together  as  sulphates,  converted  into 
carbonates  and  then  into  nitrates,  separated  by  the  ether-alcohol 
process,  and  finally  precipitated  and  weighed  as  sulphates. 

DETECTION  OF  BARIUM,   STRONTIUM,  AND  CALCIUM 
(The  process  recommended  by  Fresenius) 

When  the  alkaline  earths  are  separated  from  magnesium  — 
after  removal  of  all  other  metals  except  those  of  the  alkalies  — 
they  are  always  obtained  in  the  form  of  carbonates.     To  deter- 
mine whether  the  precipitate  consists  of  a  single  carbonate  or 


422  QUANTITATIVE  EXERCISES 

of  a  mixture  of  two  or  of  all  three  of  the  carbonates,  the  follow- 
ing method  is  recommended :  The  precipitate  is  dissolved  in 
nitric  acid,  the  solution  is  evaporated  to  dryness  in  a  small 
porcelain  dish,  and  the  residue  is  heated  in  the  dish  on  an  iron 
plate  for  10  or  15  minutes,  or  until  the  material  no  longer  smells 
of  nitric  acid,  and  a  cold  glass  plate  placed  over  the  dish  does 
not  condense  any  moisture.  The  temperature  of  the  nitrates 
may  rise  to  180°  without  danger.  The  dried  material  is  finely 
ground  with  a  pestle,  treated  with  5  cc.  of  a  mixture  of  equal 
volumes  of  anhydrous  ether  and  absolute  alcohol,  and  again 
ground.  At  the  expiration  of  a  few  minutes  the  solution  is  fil- 
tered through  a  dry  paper,  and  the  extraction  of  the  residue  is 
four  times  repeated  with  small  quantities  of  the  ether-alcohol 
mixture.  Two  drops  of  dilute  sulphuric  acid  are  added  to  the 
filtrate.  If  a  considerable  precipitate  forms,  calcium  is  present 
and  need  not  be  searched  for  further.  If,  on  the  other  hand, 
the  precipitate  is  very  small,  it  may  be  due  to  the  presence  of  a 
minute  quantity  of  strontium,  and  the  examination  for  calcium 
must  be  continued.  The  solution  is  therefore  diluted  with 
about  4  cc.  of  water  and  the  ether  and  alcohol  are  removed  by 
evaporation.  The  residual  solution  is  treated  with  a  few  drops 
of  ammonia  and  about  1  gram  of  ammonium  sulphate,  heated  to 
the  boiling  point,  and  filtered  through  a  small  paper.  The 
filtrate  is  slightly  acidified  with  acetic  acid  and  treated  with  a 
small  quantity  of  ammonium  oxalate.  If  calcium  is  present, 
a  precipitate  of  oxalate  will  form  —  immediately  if  the  quantity 
is  considerable,  but  slowly  if  it  is  minute. 

The  nitrates  of  barium  and  strontium  are  dissolved  in  100  cc. 
of  hot  water  and  the  solution  is  heated  to  the  boiling  point. 
There  are  then  added  3  or  4  drops  of  acetic  acid  and,  very 
gradually,  a  solution  of  neutral  potassium  chromate  until  the 
liquid  exhibits  a  yellow  color.  The  liquid  is  again  heated  to 
the  boiling  point.  If  the  odor  of  acetic  acid  is  perceived,  more 
chromate  is  added  until  it  disappears.  If  barium  is  present,  a 
yellow  precipitate  will  appear  —  immediately  or  after  a  short 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     423 

interval,  according  to  the  quantity  in  the  solution.  After  an 
hour  the  liquid  is  filtered,  and  a  fraction  of  the  filtrate  is  treated 
with  small  quantities  of  ammonia  and  ammonium  carbonate. 
If  a  considerable  precipitate  is  formed,  strontium  is  present.  If 
no  precipitate  appears,  or  only  a  very  small  one,  the  remainder 
of  the  filtrate  is  treated  with  1  or  2  drops  of  nitric  acid,  evap- 
orated to  a  volume  of  10  or  20  cc.,  and  treated  with  ammonia 
and  ammonium  carbonate.  If  no  precipitate  appears,  strontium 
is  absent.  A  minute  precipitate,  on  the  other  hand,  may  consist 
of  strontium  carbonate  or  possibly  of  the  calcium  salt.  Such  a 
precipitate  is  filtered,  washed,  and  dissolved  in  a  few  drops  of 
hydrochloric  acid.  The  solution  is  evaporated  to  dryness  and 
the  residue  is  dissolved  in  from  1  to  2  cc.  of  a  mixture  of  3 
parts  of  water  and  1  part  of  alcohol.  Finally,  there  is  added  to 
the  alcoholic  solution  1  drop  of  potassium  chromate,  and  the 
liquid  is  heated  just  to  the  boiling  point.  If  strontium  is  pres- 
ent, a  yellow  precipitate  will  appear,  either  immediately  or  after 
a  short  time. 

An  account  of  the  recent  important  investigation  of  Frese- 
nius  on  the  analytical  chemistry  of  the  alkaline  earths  will  be 
found  in  the  Zeitschrift  fur  analytische  Chemie,  29,  20,  143,  and 
413 ;  30,  18,  452,  and  583 ;  32,  189  and  312. 


THE  QUANTITATIVE  DETERMINATION  OF  THE 
INDIVIDUAL  ALKALINE  EARTHS 

1.  BARIUM 

1.  As  Sulphate.  To  determine  barium  as  sulphate,  the  solu- 
tion —  which  should  be  dilute  and  free  from  any  large  quantity 
of  acid  —  is  heated  to  the  boiling  point  and  treated,  with  con- 
stant stirring,  with  a  moderate  excess  of  dilute  sulphuric  acid. 
The  liquid  is  maintained  for  a  long  time  at  a  temperature  near 
the  boiling  point,  and  then  set  aside  to  settle.  The  clear  liquid 


424  QUANTITATIVE  EXERCISES 

is  poured  through  a  small  paper,  if  a  Gooch  filter  is  not  avail- 
able, and  the  residue  is  washed  several  times  by  decantation 
with  hot  water  containing  a  little  sulphuric  acid.  The  precipi- 
tate is  then  brought  upon  the  filter  and  washed  with  boiling 
water  until  the  filtrate  gives  no  reaction  for  sulphuric  acid  when 
treated  with  barium  chloride.  The  dried  precipitate  is  trans- 
ferred to  a  weighed  platinum  crucible,  the  paper  incinerated  in 
the  usual  manner,  and  the  ash  added  to  the  main  portion  of  the 
sulphate.  The  crucible  is  then  heated  for  a  long  time  over  a 
Bunsen  burner  in  order  to  reoxidize  the  barium  sulphide  formed 
during  the  burning  of  the  paper.  If  the  formation  of  oxide  is 
feared,  the  ash  may  be  moistened  with  a  very  minute  quantity 
of  sulphuric  acid  and  the  excess  of  the  acid  expelled  by  heat, 
with  or  without  the  aid  of  ammonium  carbonate. 

When  precipitated  from  a  cold  solution,  especially  in  the 
absence  of  hydrochloric  acid  or  ammonium  chloride,  the  sul- 
phate is  apt  to  pass  through  the  filter.  It  is  also  inclined  to 
carry  down  with  it  and  to  retain  with  great  tenacity  other  salts, 
particularly  the  nitrates.  For  this  reason  it  is  best  to  evapo- 
rate solutions  containing  nitrates  with  hydrochloric  acid  before 
attempting  to  precipitate  barium  as  sulphate.  The  precipita- 
tion, together  with  the  sulphate,  of  certain  substances,  notably 
iron,  can  be  largely  prevented  by  'the  addition  of  a  considerable 
quantity  of  hydrochloric  acid,  but  the  excess  of  the  sulphuric 
acid  used  to  precipitate  the  barium  must  then  be  increased  in 
order  partially  to  overcome  the  solvent  action  of  the  hydro- 
chloric acid  on  the  sulphate.  In  general,  the  solubility  of  the 
sulphate  in  hydrochloric  acid  diminishes  in  proportion  to  the 
excess  of  free  sulphuric  acid  (Fresenius).  The  salts  which  are 
precipitated  with  barium  sulphate  may  be  removed  from  it  — 
provided  their  bases  do  not  also  form  insoluble  sulphates  —  by 
dissolving  the  precipitate  in  concentrated  sulphuric  acid  and 
pouring  the  solution  into  water.  The  .second  precipitation  of 
the  sulphate  is  pure.  This  course  should  be  taken  whenever 
barium  is  precipitated  as  sulphate  for  the  purpose  of  separating 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS  425 

it  from  the  alkalies.  In  precipitating  barium  as  sulphate,  the 
quantity  of  sulphuric  acid  which  is  added  should  never  be  less 
than  will  suffice  for  the  conversion  of  all  the  bases  in  the 
solution  into  sulphates,  and  it  should  considerably  exceed  this 
amount  whenever  the  quantity  of  other  acids  is  large.  Accord- 
ing to  Fresenius,  barium  sulphate,  under  the  ordinary  conditions 
of  precipitation,  is  soluble  in  about  400,000  parts  of  water. 

2.  As  Carbonate.    The  dilute  solution  is  treated  with  ammo- 
nia and  then  with  a  moderate  excess  of  ammonium  carbonate. 
After  several  hours  the  precipitate  is  collected  upon  a  filter, 
washed  with  water  containing  a  little   ammonia,  and  ignited 
over  a  Bunsen  burner  in  a  platinum  crucible.     If  the  filter  is 
burned  in  the  crucible  without  removal  of  the  precipitate,  instead 
of  in  a  wire,  or  if  barium  in  combination  with  an  organic  acid 
is  to  be  determined  as  carbonate  by  burning  off  the  combustible 
matter,  the  residue  after  ignition  must  be  moistened  with  a  con- 
centrated solution  of  ammonium  carbonate,  the  liquid  evaporated, 
and  the  carbonate  reignited. 

Barium  carbonate  is  soluble  in  14,137  parts  of  cold  and  15,421 
parts  of  hot  water,  and  is  much  more  soluble  in  water  contain- 
ing ammonium  chloride  or  nitrate  (Fresenius).  It  is  soluble  in 
141,000  parts  of  water  containing  ammonia  and  ammonium  car- 
bonate. If  barium  is  to  be  separated  from  the  alkalies,  it  is 
better  to  precipitate  it  as  carbonate  than  as  sulphate. 

3.  As  Chromate.    The  method  by  which  barium  is  precipitated 
as  chromate  for  the  purpose  of  separating  it  from  strontium  and 
calcium  has  already  been  given.     The  process  is  not  materially 
different  when  the  other  alkaline  earths  are  absent,  except  as 
regards  the  repetition  of  the  precipitation.     The   solution   of 
the  barium  salt  is  diluted  to  about  200  cc.,  heated  to  the  boil- 
ing point,  and  treated  with  an  excess  of  ammonium  chromate. 
If  the  solution  was  previously  acid,  ammonia  is  added  until  a 
neutral  or  slightly  alkaline  reaction  is  obtained.     The  precipi- 
tate is  collected  upon  a  paper,  washed  first  with  water  contain- 
ing a  little  ammonium  chromate,  then  with  clear  cold  water, 


426  QUANTITATIVE  EXERCISES 

dried,  and  transferred  to  a  weighed  platinum  crucible.  The  paper 
is  loosely  folded,  placed  in  the  crucible,  and  burned.  The  con- 
tents of  the  crucible  are  then  heated  over  a  Bunsen  burner  until 
the  chromate  reduced  by  the  burning  paper  has  been  reoxidized. 
A  Gooch  filter  is,  of  course,  to  be  preferred  to  one  of  paper. 

The  chromate,  before  ignition,  is  soluble  in  87,000  parts  of  cold 
and  23,000  parts  of  boiling  water.  Its  solubility  is  increased  by 
the  presence  of  ammonium  salts  and  very  markedly  by  that 
of  acetic  acid.  The  ignited  salt  is  soluble  in  160,000  parts  of 
water.  In  water  containing  an  excess  of  ammonium  or  potas- 
sium chromate,  whether  ammonium  salts  are  present  or  not, 
it  is  quite  as  insoluble  as  barium  sulphate. 

When  dried  to  a  constant  weight  at  110°,  barium  chromate 
still  contains  about  0.5  per  cent  of  water.  It  can  be  gently 
ignited  without  decomposition,  but  loses  weight  at  a  bright 
red  heat.  The  chromate  which  is  reduced  by  the  burning  of 
a  filter  upon  which  it  has  been  collected  is  reoxidized  when 
heated  in  contact  with  the  air. 

4.  As  Silicofluoride.  Barium  silicofluoride  is  soluble  in  about 
3800  parts  of  cold  and  1200  parts  of  boiling  water.  Its  solu- 
bility is  greatly  increased  by  the  presence  of  hydrochloric  acid 
and  of  ammonium  salts,  and  greatly  diminished  by  the  presence 
of  alcohol,  especially  if  there  is  also  present  an  excess  of  hydro- 
fluosilicic  acid.  According  to  Fresenius,  1  part  of  the  salt  dis- 
solves: at  14.5°,  in  37,219  parts  of  a  mixture  of  equal  volumes 
of  alcohol  (96  per  cent)  and  water;  at  15°,  in  16,914  parts  of  a 
mixture  of  1  volume  of  alcohol  and  3  volumes  of  water ;  at  15°, 
in  5263  parts  of  a  mixture  of  25  volumes  of  alcohol,  74.1  vol- 
umes of  water,  and  0.9  volume  of  20  per  cent  hydrochloric  acid ; 
at  18°,  in  70,679  parts  of  a  mixture  of  25  volumes  of  alcohol, 
73  volumes  of  water,  0.9  volume  of  20  per  cent  hydrochloric 
acid,  and  1.1  volume  of  3.7  per  cent  hydrofluosilicic  acid. 

Dried  to  a  constant  weight  at  100°,  the  salt  still  contains 
a  quantity  of  water  which  varies  somewhat  according  to 
the  conditions  under  which  it  was  precipitated.  At  elevated 


METALS  OF  THE  ALKALIES  AM)  ALKALINE  EARTHS     427 

temperatures  it  is  decomposed.  It  is  therefore  not  practicable 
to  determine  barium  by  weighing  the  silicofluoride.  The  silico- 
fluoride  may,  however,  be  converted  into  sulphate  in  the  fol- 
lowing manner :  The  precipitate,  after  being  washed  and  dried, 
is  removed  from  the  filter.  The  filter  is  burned  in  a  platinum 
crucible,  and  to  the  ash  is  added  the  precipitate.  The  contents 
of  the  crucible  are  treated  with  concentrated  sulphuric  acid  and 
heated,  first  upon  a  water  bath  and  then  upon  a  sand  bath,  to 
expel  the  excess  of  sulphuric  acid.  The  residue  is  treated  with 
hydrofluoric  and  sulphuric  acids  and  again  heated  to  remove 
any  silica  which  may  have  separated  during  the  decomposition 
of  the  silicofluoride. 

The  precipitation  of  barium  as  silicofluoride,  rather  than  as 
sulphate,  carbonate,  or  chromate,  will  not  be  found  advanta- 
geous except  as  a  means  of  effecting  a  separation  from  stron- 
tium and  calcium ;  and  even  for  this  purpose  the  silicofluoride 
process  is  distinctly  inferior  to  the  chromate  method. 

2.  STRONTIUM 

1.  As  Sulphate.  The  rather  concentrated  solution  of  the  stron- 
tium salt,  which  should  be  free  from  any  considerable  quantity 
of  hydrochloric  or  nitric  acid,  is  treated  with  an  excess  of  dilute 
sulphuric  acid,  and  then  with  not  less  than  an  equal  volume  of 
alcohol.  After  12  hours  the  precipitate  is  collected  upon  a  filter 
and  washed  with  a  mixture  of  equal  volumes  of  alcohol  and 
water  until  a  neutral  filtrate  is  obtained.  The  precipitate  must 
be  removed  as  perfectly  as  possible  from  the  filter  before  incin- 
erating the  latter,  and  the  former  must  be  thoroughly  dried  at 
a  moderate  temperature  before  the  final  ignition.  The  first  pre- 
caution is  rendered  necessary  by  the  volatility  of  strontium  com- 
pounds in  the  flame  of  a  Bunsen  burner,  and  the  second  by 
the  liability  of  the  sulphate,  when  not  fully  dried,  to  suffer  loss 
in  weight  in  consequence  of  mechanical  transportation  by  the 
escaping  vapors. 


428  QUANTITATIVE  EXERCISES 

Strontium  cannot,  of  course,  be  precipitated  as  sulphate  in 
the  manner  described  when  the  solution  contains  other  sub- 
stances which  are  insoluble  or  very  difficultly  soluble  in  from 
40  to  50  per  cent  alcohol.  In  such  cases  the  strontium  should 
be  precipitated  as  carbonate  as  directed  under  2. 

Strontium  sulphate  is  soluble  in  6895  parts  of  cold  and  in 
9638  parts  of  boiling  water  (Fresenius).  In  water  containing 
free  sulphuric  acid  it  is  decidedly  less  soluble  (11,862  parts  of 
cold  water)  than  in  pure  water.  Its  solubility  is  greatly  in- 
creased by  the  presence  of  hydrochloric  or  nitric  acid,  but  not 
by  that  of  acetic  acid.  Chlorides  of  the  alkalies  and  also  of 
magnesium  increase  its  solubility  in  water ;  hence  the  quantity 
of  sulphuric  acid  which  is  added  for  the  purpose  of  precipitating 
strontium  should  always  be  sufficient  to  convert  all  chlorides 
of  other  metals  in  the  solution  into  sulphates.  In  50  per  cent 
alcohol  strontium  sulphate  is  nearly  insoluble.  Like  barium 
sulphate,  the  strontium  salt  is  inclined,  though  to  a  less  extent 
than  the  former,  to  carry  down  with  it  and  to  retain  other  com- 
pounds. It  is  stable  at  a  red  heat  and  is  completely  converted 
into  carbonate  at  ordinary  temperatures  by  ammonium  or  other 
alkaline  carbonates.  The  conversion  into  carbonate  is  complete 
notwithstanding  the  presence  of  considerable  quantities  of  the 
sulphate  of  the  alkalies,  and  is  hastened  by  the  application  of 
heat.  The  difference  between  barium  and  strontium  sulphates 
in  respect  to  their  conduct  towards  the  carbonates  of  the  alka- 
lies does  not,  however,  afford  a  means  for  the  quantitative  sepa- 
ration of  the  two  metals.  If  the  mixed  sulphates  are  treated 
with  ammonium  carbonate  or  with  a  solution  of  potassium  car- 
bonate and  sulphate,  a  portion  of  the  strontium  sulphate  is  not 
converted  into  carbonate  when  the  barium  sulphate  is  in  excess, 
while,  on  the  other  hand,  some  of  the  barium  sulphate  is  changed 
into  carbonate  when  the  strontium  sulphate  predominates  in  the 
mixture. 

2.  As  Carbonate.  The  dilute  solution  of  the  strontium  salt 
is  treated  with  ammonia  and  then  with  a  moderate  excess  of 


METALS  OF  THE  ALKALIES  AXD  ALKALINE  EARTHS  429 

ammonium  carbonate.  After  some  hours  the  precipitate  is 
collected  upon  a  filter,  washed  with  water  containing  a  little 
ammonia,  and  ignited. 

Strontium  carbonate  is  soluble  in  18,045  parts  of  water  at 
ordinary  temperatures,  and  in  56,545  parts  of  water  to  which 
ammonia  and  ammonium  carbonate  have  been  added  (Frese- 
nius).  It  is  dissolved  to  a  considerable  extent  by  ammonium 
chloride  and  nitrate,  but  is  re  precipitated  from  such  solutions 
by  ammonia  and  ammonium  carbonate. 

3.  CALCIUM 

1.  As  Sulphate.  The  solution,  which  must  contain  no  salts 
insoluble  in  60  or  70  per  cent  alcohol,  is  treated  with  a  slight 
excess  of  dilute  sulphuric  acid,  and  then  with  three  or  four 
times  its  volume  of  alcohol.  After  12  hours  the  precipitate  is 
collected  upon  a  filter,  washed  with  alcohol,  and  heated  to  a 
dull  red.  If  a  paper  has  been  used  as  a  filter,  the  ash  should 
be  moistened  with  sulphuric  acid  or  with  a  solution  of  ammo- 
nium sulphate  before  the  final  ignition. 

Calcium  sulphate  is  soluble  in  430  parts  of  cold  and  in  460 
parts  of  boiling  water.  Hydrochloric  and  nitric  acids,  also  am- 
monium chloride,  sodium  sulphate  and  chloride,  increase  its  sol- 
ubility in  water.  It  is  quite  readily  soluble  in  hot  solutions  of 
ammonium  sulphate  and  sodium  thiosulphate,  in  which  the  cor- 
responding barium  and  strontium  salts  are  insoluble.  Methods 
for  the  separation  of  calcium  from  barium  and  strontium,  based 
on  this  difference  in  the  conduct  of  the  sulphates,  have  not, 
however,  proved  satisfactory.  Like  the  corresponding  salt  of 
strontium,  calcium  sulphate  is  readily  converted  into  carbonate 
by  solutions  of  the  carbonates  of  the  alkalies,  but  calcium  can- 
not thereby  be  separated  from  barium,  whose  sulphate  is  not 
attacked  by  alkaline  carbonates. 

Calcium  sulphate  is  stable  at  a  low  red  heat,  but  at  higher 
temperatures  it  is  converted  into  oxide. 


430  QUANTITATIVE  EXERCISES 

2.  As  Carbonate,  through  Precipitation  with  Ammonium  Car- 
bonate.   The  solution  is  treated  with  ammonia  and  an  excess  of 
ammonium  carbonate,  and  the  precipitate,  after  collection  upon 
a  filter,  is  washed  with  water  to  which  ammonia  has  been  added. 
The  carbonate  is  only  gently  ignited,  owing  to  the  danger  of 
converting  it  into   oxide,   and  before   weighing  it  should   be 
moistened  with  a  concentrated  solution  of  ammonium  carbon- 
ate, slowly  dried,  and  finally  heated  to  a  temperature  just  suffi- 
cient for  the  volatilization  of  the  ammonium  salt. 

Calcium  carbonate  is  soluble  in  28,500  parts  of  water  and  in 
65,000  parts  of  water  containing  ammonia  and  ammonium  car- 
bonate. It  is  more  soluble  in  water  containing  ammonium  salts, 
but  is  reprecipitated  from  such  solutions  by  ammonia  and 
ammonium  carbonate.  Neutral  salts  of  potassium  and  sodium, 
also  of  calcium  and  magnesium,  increase  somewhat  its  solu- 
bility. It  is  stable  at  a  low  red  heat,  but  is  converted  into  oxide 
at  higher  temperatures. 

3.  As  Carbonate  or  Oxide,  through  Precipitation  by  Ammo- 
nium Oxalate.    The  hot  solution  of  the  calcium  salt  is  treated 
with  an  excess  of  ammonium  oxalate  and  then  with  ammonia  to 
alkaline  reaction.     After  standing  in  a  warm  place  for  not  less 
than  12  hours  —  at  all  events  until  the  supernatant  solution  is 
perfectly  clear  —  the  liquid  is  poured  through  a  filter  without 
disturbing  the  precipitate  in  the  bottom  of  the  vessel.     The 
residue  is  washed  several  times  by  decantation  with  hot  water — 
care  being  taken  to  allow  each  portion  of  the  liquid  to  pass 
through  the  paper  before  adding  another — and  then  brought 
upon  the  filter.     If  portions  of  the  precipitate  adhere  to  the 
glass  so  firmly  that  they  cannot  be  detached,  they  are  dissolved 
in  a  little  hydrochloric  acid,  the  solution  is  transferred  to  a  small 
vessel,  and  the  oxalate  reprecipitated  by  the  addition  of  ammo- 
nia.    After  settling,  the  second  precipitate  is  brought  upon  the 
filter  in  the  same  manner  as  the  first.     The  oxalate  is  washed 
with  hot  water,  dried,  and  removed  to  a  weighed  platinum  cru- 
cible.    The  filter  paper  is  burned  in  a  platinum  wire  and  the 


METALS  OF  THE  ALKALIES  AND  ALKALINE  EARTHS     431 

ash  is  also  placed  in  the  crucible.  The  oxalate  is  heated  very 
moderately  for  a  long  time  and  afterwards  to  a  dull  red  heat, 
until  the  mass,  on  cooling,  appears  perfectly  white.  The  mate- 
rial, which  may  now  contain  some  oxide,  is  moistened  with  a 
concentrated  solution  of  ammonium  carbonate,  slowly  dried,  and 
very  gently  ignited.  The  treatment  with  ammonium  carbon- 
ate, etc.,  must  be  repeated  until  a  constant  weight  is  obtained. 
It  is  practicable,  though  not  advantageous,  to  convert  the  car- 
bonate into  and  to  weigh  the  calcium  as  oxide. 

Precipitated  calcium  oxalate  is  exceedingly  finely  divided, 
and  is  apt  to  give  trouble  by  passing  through  the  filter.  It  is 
for  this  reason  that  the  oxalate  is  never  filtered  until  the  pre- 
cipitate has  completely  subsided,  and  that  it  is  washed  as  far  as 
practicable  by  decantation.  Its  solubility  in  water,  which  is  very 
slight,  is  not  affected  by  the  presence  of  the  chlorides  of  the 
alkalies  or  of  the  alkaline  earths.  Magnesium  chloride,  on  the 
other  hand,  increases  its  solubility  to  a  marked  degree ;  hence  in 
precipitating  calcium  by  ammonium  oxalate,  enough  of  the  lat- 
ter should  be  added  also  to  convert  any  magnesium  which  may 
be  present  into  the  oxalate.  Dried  to  a  constant  weight  at  100°, 
the  salt  still  contains  1  molecule  of  water  of  crystallization. 

Calcium  oxalate  may  be  determined,  either  directly  or  indi- 
rectly, by  potassium  permanganate.  To  determine  it  directly,  the 
well-washed  and  still  moist  precipitate  is  returned  to  the  beaker 
and  treated  with  an  excess  of  dilute  sulphuric  acid.  The  solu- 
tion is  diluted  to  150  or  200  cc.,  heated  to  60°,  and  then  titrated 
to  color  with  a  dilute  standard  solution  of  potassium  perman- 
ganate. To  determine  it  indirectly,  the  calcium  salt  is  treated 
in  a  graduated  flask  with  a  measured  but  excessive  quantity  of 
a  standard  solution  of  oxalic  acid,  and  then  with  ammonia  to  alka- 
line reaction.  The  liquid  is  heated  to  the  boiling  point  and  then 
allowed  to  cool.  The  flask  is  filled  to  the  mark  with  water, 
shaken  well,  and  its  contents  filtered  through  a  dry  paper. 
Finally,  measured  portions  of  the  filtrate  are  heated  to  60°,  acid- 
ified with  sulphuric  acid,  and  titrated  with  permanganate. 


CHAPTER   XIX 
THE  ALKALINE  EARTHS  (CONTINUED)  AND  MAGNESIUM 

THE   SEPARATION   OF  THE  ALKALINE  EARTHS  FROM 

THE   ACIDS 

The  only  acids  which  require  mention  in  this  connection  are 
those  with  which  the  metals  form  insoluble  salts  or  those  which 
must  be  removed  before  the  metals  can  be  precipitated  from 
their  solutions  by  some  particular  reagent. 

1.  SULPHURIC  ACID 

The  sulphates  of  strontium  and  calcium,  when  free  from 
barium  sulphate,  are  readily  and  completely  decomposed  at 
ordinary  temperatures  by  neutral  ammonium  carbonate.  All 
three  of  the  sulphates  are  converted  into  carbonates  by  a  boil- 
ing solution  of  sodium  carbonate.  But  in  the  case  of  the 
barium  salt  the  decomposition  cannot  be  completed  in  the  pres- 
ence of  any  considerable  quantity  of  a  soluble  sulphate;  hence 
it  is  necessary,  after  boiling  for  a  time,  to  pour  off  the  liquid 
and  to  repeat  the  treatment  with  fresh  portions  of  the  alkaline 
carbonate.  It  is  better,  however,  when  sulphuric  acid  is  to  be 
separated  from  the  alkaline  earths,  to  mix  the  finely  pulverized 
sulphates  in  a  platinum  crucible  with  four  or  five  times  their 
weight  of  potassium-sodium  carbonate,  and  to  fuse  the  mixture. 
If  the  fused  mass  is  treated  with  hot  water  until  all  soluble  mat- 
ter is  dissolved,  and  the  insoluble  carbonates  are  collected  upon 
a  filter  and  thoroughly  washed,  the  separation  of  the  sulphuric 
acid  is  complete. 


THE  ALKALINE  EARTHS  AND  MAGNESIUM          433 

2.  PHOSPHORIC  ACID 

Whether  the  presence  of  phosphoric  acid  is  objectionable  or 
not  depends  upon  the  form  in  which  it  is  proposed  to  precipi- 
tate the  alkaline  earths.  If  they  are  to  be  precipitated  from 
acid  solutions  as  sulphates,  the  phosphoric  acid  may  remain; 
while  if  the  precipitation  is  to  be  made  in  a  neutral  or  alkaline 
solution,  e.g.  as  carbonate,  oxalate,  chromate,  etc.,  it  must  be 
removed. 

Unlike  sulphuric  acid,  phosphoric  acid  cannot  be  quantita- 
tively separated  from  barium,  strontium,  and  calcium,  either  by 
boiling  with  a  solution  of  an  alkaline  carbonate  or  by  fusion 
with  the  dry  carbonates.  The  separation  may,  however,  be 
effected  by  precipitation  of  the  acid  in  various  ways. 

1.  The  phosphates  are  dissolved  in  the   slightest  possible 
excess  of  nitric   acid.     The  solution  is   treated  with  a  little 
ammonium  chloride,  then  with  lead  acetate  as  long  as  a  precipi- 
tate forms,  and  finally  with  precipitated  lead  carbonate.     After 
digesting  for  some  time,  the  precipitate  is  collected  and  washed. 
The  filtrate,  which  contains  the  alkaline  earths,  also  the  alka- 
lies if  any  were  present,  and  some  lead  salts,  is  treated  with  a 
rapid  current  of  hydrogen  sulphide  and  quickly  filtered. 

2.  The  phosphates  are  dissolved,  as  in  the  preceding  case, 
with  the  least  possible  excess  of  nitric  acid.     The  filtrate  is 
treated  with  silver  nitrate,  and  then  with  silver  carbonate  until 
a  neutral  reaction  is  obtained.     The  precipitate  is  collected  and 
washed,  and  the  silver  in  the  filtrate  is  removed  by  hydrochlo- 
ric acid.     This  process,  like  the  preceding  one,  separates  the 
alkalies  as  well  as  the  alkaline  earths  from  phosphoric  acid. 

3.  The  phosphates  are  dissolved  in  a  porcelain  dish  with 
nitric  acid.     There  is  then  added  a  quantity  of  pure  mercury 
which  is  somewhat  greater  than  will  dissolve  in  the  excess  of 
nitric  acid.     The  solution  is  evaporated  to  complete  dryness 
upon  a  water  bath.     The  residue  is  moistened  with  a  very  small 
quantity  of  water  and  again  dried,  and  the  operation  is  repeated 


434  QUANTITATIVE  EXERCISES 

until  no  clouds  form  when  a  glass  rod  wet  with  ammonia  is  held 
over  the  dish.  The  residue  is  digested  with  either  hot  or  cold 
water,  collected  upon  a  filter,  and  thoroughly  washed.  The 
filtrate  contains,  in  addition  to  the  metals  from  which  the  phos- 
phoric acid  has  been  separated,  a  small  quantity  of  mercury 
compounds  which  must  be  removed.  To  this  end  the  solution 
is  evaporated  to  dryness  in  a  platinum  dish  and  the  residue  is 
very  gently  ignited  for  a  long  time.  If  metals  of  the  alkalies 
are  present,  the  contents  of  the  dish  should  be  treated  from 
time  to  time  with  pieces  of  solid  ammonium  carbonate  to  pre- 
vent the  formation  of  alkaline  oxides  from  the  nitrates,  which 
would  attack  the  platinum. 

This  method  of  Rose  effects  a  satisfactory  separation  of  phos- 
phoric acid,  not  only  from  the  alkaline  earths  and  the  alkalies, 
but  also  from  all  other  metals  except  iron,  aluminium,  and 
mercury. 

3.  HYDROFLUORIC  ACID 

The  only  compound  which  need  be  considered  in  this  place 
is  the  frequently  occurring  mineral,  calcium  fluoride,  which  pre- 
sents some  analytical  difficulties. 

1.  The  usual  method  of  separating  fluorine  from  calcium, 
when  a  direct  determination  of  the  former  is  not  to  be  attempted, 
is  as  follows :  The  pulverized  material  is  treated  in  a  platinum 
crucible  with  an  excess  of  concentrated  sulphuric  acid,  which  is 
afterwards  evaporated.  The  fluorine  escapes  as  hydrofluoric 
acid  while  the  calcium  remains  as  sulphate.  This  method  of 
separating  fluorine  from  a  metal  is  applicable  to  many  other 
fluorides  than  the  calcium  salt,  but  not  to  all.  Some  fluorides 
containing  aluminium  resist  the  action  of  concentrated  sulphuric 
acid.  Such  substances  may,  however,  be  decomposed  by  fusion 
with  acid  potassium  sulphate. 

If  a  fluoride  contains  silica  or  a  silicate,  or  if  one  of  these  is 
mingled  with  it,  it  is  decomposed  by  concentrated  sulphuric 
acid  with  evolution  of  silicon  fluoride  instead  of  hydrofluoric 


THE  ALKALINE  EARTHS  AND  MAGNESIUM          435 

acid.     This  fact  is  often  utilized  for  the  separation  and  also  for 
the  determination  of  fluorine. 

2.  If  calcium  fluoride  is  heated  with  a  dry  alkaline  carbonate, 
the  mixture  fuses  easily  to  a  clear  liquid.  But  on  treating  the 
mass  with  water,  the  compound  is  found  to  be  only  slightly 
decomposed.  But  if  calcium  fluoride  is  fused  with  the  carbonate 
in  the  presence  of  silica,  the  decomposition  is  complete  and  the 
solution  obtained  by  treating  the  mass  with  water  contains  all 
the  fluorine  and  also  the  silica  in  combination  with  an  alkali, 
while  the  calcium  remains  as  carbonate. 

THE  SEPARATION   OF  THE   ALKALINE  EARTHS  FROM 
THE  ALKALIES 

The  dilute  solution  —  which  should  be  nearly  free  from 
ammonium  salts  —  is  treated  with  a  little  ammonia  and  then 
with  a  slight  excess  of  ammonium  carbonate.  After  standing 
for  an  hour  or  more  in  a  warm  place,  the  precipitate  is  collected 
upon  a  filter  and  washed  with  water  to  which  has  been  added  a 
little  ammonia. 

If  magnesium  is  also  present,  as  usually  happens  in  practice, 
enough  ammonium  chloride  must  be  added  to  the  solution  to 
prevent  the  precipitation  of  magnesium  carbonate. 

The  carbonates  of  the  alkaline  earths  are  somewhat  soluble  in 
water  containing  ammonium  chloride  or  nitrate,  though  much 
less  so  in  the  presence  of  ammonia  and  ammonium  carbonate. 
Of  the  three,  barium  carbonate  is  the  most,  and  strontium  car- 
bonate the  least,  soluble.  The  filtrate  may  therefore  contain, 
in  addition  to  the  alkalies,  weighable  quantities  of  barium  and 
calcium.  To  remove  these,  the  filtrate  is  treated  with  3  or  4 
drops  of  dilute  sulphuric  acid,  and  somewhat  later  with  a  little 
ammonium  oxalate.  After  standing  for  12  hours  in  a  warm 
place,  the  precipitate,  if  one  appears,  is  collected  and  washed 
with  water.  The  residue  on  the  filter  will  contain  the  barium 
as  sulphate  and  the  calcium  as  oxalate,  and  it  may  also  contain 


436  QUANTITATIVE  EXERCISES 

a  little  magnesium  oxalate.  It  is  treated  with  hydrochloric  acid 
and  the  undissolved  barium  sulphate  is  washed  with  water. 
The  filtrate  is  neutralized  with  ammonia  to  reprecipitate  the 
calcium  and  to  separate  it  from  the  small  quantity  of  magne- 
sium which  may  be  present.  The  calcium  oxalate,  after  stand- 
ing the  usual  length  of  time,  is  collected  and  washed  and  the 
nitrate  is  added  to  that  which  contains  the  alkalies  and  the 
greater  portion  of  the  magnesium.  The  small  quantities  of 
barium  and  calcium  recovered  in  this  way  are  determined  inde- 
pendently, while  the  main  body  of  the  alkaline  earths  is  treated 
as  directed  under  Exercise  XLIX. 

If  magnesium  is  absent,  the  alkalies  are  isolated  in  the  form 
of  chlorides  and  separated  in  the  usual  manner  by  means  of 
platinum  chloride.  If  magnesium  is  present,  it  must  be  removed 
before  proceeding  to  the  separation  and  determination  of  potas- 
sium and  sodium. 

The  method  just  given  provides  for  the  separation  from  the 
alkalies  of  any  one  or  all  of  the  alkaline  earths.  Barium  and 
calcium,  if  either  occurs  without  the  other  and  without 
strontium,  may  also  be  separated  from  the  alkalies  by  other 
methods.  If  of  the  alkaline  earths  barium  only  is  present,  the 
solution  is  treated  with  a  quantity  of  dilute  sulphuric  acid 
which  will  suffice  for  the  precipitation  of  the  barium  and  also 
for  the  conversion  of  the  metals  of  the  alkalies  into  sulphates. 
The  precipitate  is  collected,  washed,  dried,  dissolved  in  concen- 
trated sulphuric  acid,  and  reprecipitated  by  dilution  with  water. 
The  solution  and  reprecipitation  of  the  barium  sulphate  are 
necessary  in  order  to  recover  the  alkalies  which  are  carried 
down  during  the  first  precipitation  and  which  cannot  be  removed 
from  the  precipitate  by  washing.  If  only  calcium  is  present, 
the  separation  may  be  effected  by  precipitation  with  ammonium 
oxalate  in  the  usual  manner. 


THE  ALKALINE  EARTHS  AND  MAGNESIUM  437 


THE  DETERMINATION  OF  MAGNESIUM 

Magnesium  is  usually  determined  as  pyrophosphate  in  the 
following  manner:  The  solution  is  treated  with  ammonium 
chloride  and  then  with  an  excess  of  ammonia.  If  a  precipitate 
appears,  it  is  dissolved  by  adding  more  ammonium  chloride,  or 
it  is  dissolved  with  hydrochloric  acid  and  the  solution  is  again 
made  alkaline  with  ammonia.  The  clear  liquid  is  then  treated 
with  sodium  phosphate,  care  being  taken  while  stirring  not  to 
allow  the  stirrer  to  come  in  contact  with  the  beaker  glass. 
After  12  hours  the  precipitate  is  brought  upon  a  filter  —  with 
the  aid  of  portions  of  the  filtrate  —  and  washed  with  a  mixture 
of  3  parts  of  water  and  1  part  of  ammonia  (sp.  gr.  .96)  until 
the  filtrate  gives  no  reaction  for  chlorine.  The  precipitate  is 
removed  as  perfectly  as  possible  from  the  filter  to  a  porcelain 
crucible.  The  paper  is  burned  in  a  platinum  wire  and  the  ash 
added  to  the  main  portion  of  the  phosphate.  The  crucible  is 
heated  for  a  long  time  at  a  moderate  temperature,  afterwards 
to  redness,  and  finally  over  the  blast  lamp.  If  the  pyrophos- 
phate has  a  gray  color,  it  should  be  treated  with  nitric  acid 
and  reheated. 

If  it  is  desired  to  avoid  the  presence  of  sodium  in  the  filtrate, 
a  solution  of  ammonium  phosphate  (H(NH4)2PO4)  or  of  phos- 
phoric acid  may  be  employed  to  precipitate  the  magnesium. 

The  precipitated  salt  has  the  composition  NH4MgPO4  •  6  H2O, 
which  on  ignition  yields  Mg2P2O7.  It  is  soluble  in  about 
15,000  parts  of  cold  water,  but  is  much  less  soluble  in  water 
containing  ammonia.  Its  solubility,  even  in  water  containing 
ammonia,  is  increased  by  the  presence  of  ammonium  chloride 
and  diminished  by  that  of  a  soluble  phosphate. 

If  a  solution  contains  no  other  nonvolatile  matter  than  mag- 
nesium, the  latter  may  be  determined  as  sulphate  by  evaporating 
with  sulphuric  acid  and  gently  igniting  the  residue. 


438  QUANTITATIVE  EXERCISES 

THE  SEPAKATION  OF  MAGNESIUM  FROM  THE 
ALKALIES 

The  method  here  recommended  is  applicable  only  to  the  chlo- 
rides. If,  therefore,  the  metals  are  not  already  in  the  form  of 
chlorides,  or  if  other  acids  than  hydrochloric  acid  are  present, 
the  solution  is  evaporated  to  dryness  in  a  porcelain  crucible  and 
the  residue  is  heated  repeatedly  with  small  portions  of  ammo- 
nium chloride,  care  being  taken,  in  the  final  heating  at  least, 
to  volatilize  all  the  excess  of  the  ammonium  salt.  The  dried 
material,  consisting  of  chlorides  and  possibly  of  some  magne- 
sium oxide,  is  treated  with  a  little  warm  water  and  then  with  a 
quantity  of  pure  mercuric  oxide  suspended  in  water  which  is 
somewhat  more  than  sufficient  to  precipitate  all  the  magnesium  as 
oxide.  The  crucible  is  heated  on  the  water  bath  until  the  con- 
tents appear  to  be  dry,  and  then  for  a  long  time  to  a  somewhat 
higher  temperature.  The  crucible  is  afterwards  covered  and 
more  strongly  heated  in  a  good  draught  until  all  of  the  mercuric 
chloride  has  been  expelled.  The  residue  consists  of  the  chlorides 
of  the  alkalies,  magnesium  oxide,  and  some  mercuric  oxide.  It 
is  treated  with  hot  water  and  filtered.  The  insoluble  matter  is 
washed  with  hot  water  and  dried.  The  paper,  without  removal 
of  the  contents,  is  rolled  up  and  burned  in  a  weighed  crucible. 
Finally,  the  magnesium  oxide  is  heated  to  expel  the  remaining 
mercuric  oxide.  Care  must,  of  course,  be  taken  to  avoid  inhaling 
the  poisonous  vapors  of  mercury  and  its  compounds. 

The  filtrate  contains  the  alkalies  as  chlorides,  and  these  are 
separated  and  determined  in  the  usual  manner. 

OTHER  METHODS  OF  SEPARATING  MAGNESIUM  FROM  THE 

ALKALIES 

1.  By  Means  of  Ammonium  Phosphate.  The  solution  is  treated 
with  ammonium  chloride,  an  excess  of  ammonia,  and  a  slight 
excess  of  ammonium  phosphate.  The  magnesium  is  precipitated 


THE  ALKALINE  EARTHS  AM)  MAGNESIUM          439 

as  ammonium-magnesium  phosphate,  and  is  weighed  as  the 
pyrophosphate.  The  filtrate  contains  the  alkalies  and  also 
phosphoric  acid,  which  must  be  removed  by  one  of  the  methods 
already  given,  before  the  potassium  and  sodium  can  be  separated 
and  determined. 

2.  By  Means  of  Barium  Hydroxide.    The  neutral  solution  — 
which  must  be  free  from  ammonium  salts — is  treated  with  a 
solution  of  barium  hydroxide  as  long  as  any  precipitate  forms, 
and  is  then  heated  to  the  boiling  point.     The  precipitate,  consist- 
ing of  magnesium  hydroxide  and  possibly  also  of  barium  sul- 
phate and  carbonate,  is  collected  upon  a  filter  and  washed  with 
boiling  water.     It  is  then  dissolved  in  hydrochloric  acid,  and 
the  barium  is  precipitated  as  sulphate  and  the  magnesium  as 
ammonium-magnesium  phosphate. 

The  filtrate  contains  the  alkalies,  the  soluble  compounds  of 
barium,  and,  owing  to  the  solubility  of  the  hydroxide  in  water  con- 
taining salts  of  the  alkalies,  a  small  quantity  of  magnesium.  The 
barium  is  precipitated  as  carbonate  or  sulphate  while  the  magne- 
sium is  recovered  by  the  mercuric  oxide  method,  or  neglected, 
according  as  exact  or  only  approximate  results  are  required. 

3.  By  Means  of  Calcium  Hydroxide.    The  solution  is  treated 
with  pure  calcium  hydroxide  suspended  in  water,  and  heated 
to  the  boiling  point.    After  filtering  and  washing,  the  calcium 
and  magnesium  in  the  precipitate  are  separated  by  means  of 
ammonium  oxalate.     The  calcium  in  the  filtrate  is  separated 
from  the  alkalies  by  precipitation  as  carbonate  or  as  oxalate.     A 
small  portion  of  the  magnesium  will  be  found  with  the  alkalies, 
owing  to  the  effect  of  the  salts  of  the  latter  upon  the  hydroxide 
of  the  former. 

There  are  still  other  methods  of  separating  magnesium  from 
the  alkalies.  One  of  them  is  based  on  the  fact  that,  under  cer- 
tain conditions,  magnesium  can  be  precipitated  as  a  double 
ammonium-magnesium  carbonate ;  another  upon  the  fact  that 
when  magnesium  ^chloride  is  heated  with  oxalic  acid  it  is 
converted  into  oxide. 


440  QUANTITATIVE  EXERCISES 

THE  SEPARATION  OF  MAGNESIUM  AND  BAEIUM 

1.  By  Precipitation  of  the  Barium  as  Carbonate.    The  dilute 
solution  is  treated  with  ammonium  chloride  until  ammonia  gives 
no  precipitate,  then  with  an  excess  of  ammonia,  and  finally  with  a 
slight  excess  of  ammonium  carbonate.     After  standing  (covered) 
for  an  hour  in  a  moderately  warm  place,  the  liquid  is  filtered  and 
the  precipitate  is  washed  with  water  containing  a  little  ammonia. 

The  barium  carbonate  upon  the  filter  may  contain  a  little 
ammonium-magnesium  carbonate.  It  is  therefore  dissolved  in 
hydrochloric  acid,  and  the  barium  is  precipitated  as  sulphate  in 
the  usual  manner.  The  filtrate  from  the  sulphate  is  evaporated 
to  dryness,  the  residue  dissolved  in  water,  and  the  little  mag- 
nesium in  the  solution  is  precipitated  as  ammonium-magnesium 
phosphate. 

The  first  filtrate  —  that  containing  the  greater  portion  of  the 
magnesium  —  contains  a  little  barium  owing  to  the  solubility  of 
barium  carbonate  in  the  presence  of  ammonium  salts.  It  is  there- 
fore treated  with  3  or  4  drops  of  dilute  sulphuric  acid  and  the 
minute  precipitate  is  collected  upon  the  same  filter  with  the  larger 
quantity  of  barium  sulphate.  The  magnesium  in  the  filtrate  is 
precipitated  as  ammonium-magnesium  phosphate,  which  is  col- 
lected with  that  recovered  from  the  barium  carbonate. 

2.  By  Precipitation  of  the  Barium  as  Sulphate.    The  barium  is 
precipitated  as  sulphate  in  the  usual  manner,  and  the  magnesium 
in  the  filtrate  is  precipitated  as  ammonium-magnesium  phosphate. 
'""  If  barium  and  magnesium  are  the  only  metals  to  be  separated, 
i.e.   if  other  alkaline  earths   and   the  alkalies  are  absent,  the 
second  method  is  to  be  preferred  to  the  first. 

THE  SEPAKATION  OF  MAGNESIUM  FEOM  STRONTIUM 

1.  By  Precipitation  of  the  Strontium  as  Carbonate.  The  pro- 
cedure is  precisely  the  same  as  for  the  separation  of  magnesium 
and  barium.  It  is  to  be  remarked,  however,  that  the  filtrate 


THE  ALKALINE  EARTHS  AND  MAGNESIUM  441 

from  the  carbonate  contains  only  traces  of  strontium,  and  it 
need  not  therefore  be  treated  with  sulphuric  acid,  or  rather, 
with  sulphuric  acid  and  alcohol. 

2.  By  Precipitation  of  the  Strontium  as  Sulphate.  There  are 
added  to  the  solution  of  the  salts  of  the  two  metals  one-half  its 
volume  of  alcohol,  and  a  quantity  of  dilute  sulphuric  acid  which 
will  more  than  suffice  to  convert  both  into  sulphates.  After  12 
hours  the  precipitate  is  collected  on  a  filter  and  washed  with 
water  to  which  has  been  added  one-third  its  volume  of  alcohol. 
The  strontium  is  weighed  as  sulphate.  The  alcohol  in  the  fil- 
trate is  evaporated  at  a  low  temperature  and  the  magnesium  is 
precipitated  as  ammonium-magnesium  phosphate. 

THE  SEPARATION  OF  MAGNESIUM  FROM  CALCIUM 

1.  By  Precipitation  of  the  Calcium  as  Carbonate.    As  far  as  the 
precipitation  of  the  calcium  is  concerned,  the  procedure  is  the 
same  as  in  the  separation  of  magnesium  from  barium  or  stron- 
tium.    To  recover  the  small  quantity  of  magnesium  which  may 
be  precipitated  with  the  calcium  carbonate,  the  precipitate  is 
dissolved  and  the  calcium  reprecipitated  either  by  ammonium 
oxalate  or  by  sulphuric  acid  and  alcohol.     Since  calcium  car- 
bonate is  somewhat  soluble  in  water  containing  ammonium  chlo- 
ride, the  filtrate  —  the  first  one  —  will  contain  a  little  calcium. 
To  recover  this,  the  filtrate  is  treated  with  enough  ammonium 
oxalate  to  convert  both  calcium  and  magnesium  into  oxalates. 
After  12  hours  the  precipitate  is  collected  and  washed.    It  con- 
sists mainly  of  calcium  oxalate,  but  may  contain  a  little  mag- 
nesium oxalate.     To  remove  this,  the  precipitate  is  dissolved 
in  hydrochloric  acid,  and  the  calcium  is  reprecipitated  by  adding 
ammonia  and  a  small  quantity  of  ammonium  oxalate. 

2.  By  Precipitation  of  the  Calcium  as  Oxalate.   The  dilute  solu- 
tion is  treated  with  ammonium  chloride  until  ammonia  gives  no 
precipitate,  then  with  a  slight  excess  of  ammonia,  and  finally 
with  enough  ammonium  oxalate  to  convert  both  magnesium  and 


442  QUANTITATIVE  EXERCISES 

calcium  into  oxalates.  If  less  of  the  last  reagent  is  added,  the 
precipitation  of  the  calcium  will  be  incomplete,  owing  to  the 
solubility  of  its  oxalate  in  a  solution  of  magnesium  chloride. 
After  12  hours  the  precipitate  is  collected  and  washed.  It  will 
contain  a  little  magnesium  oxalate.  It  is  therefore  dissolved 
in  hydrochloric  acid  and  the  calcium  reprecipitated  by  adding 
ammonia  and  a  small  quantity  of  ammonium  oxalate. 

The  two  nitrates,  which  contain  all  the  magnesium  and  no 
weighable  quantity  of  calcium,  are  evaporated  to  dryness  in  a 
platinum  dish  and  the  ammonium  salts  are  expelled  by  heat. 
The  nonvolatile  residue  is  dissolved  in  hydrochloric  acid,  and 
the  magnesium  is  precipitated  in  the  usual  manner  by  sodium 
or  ammonium  phosphate. 

THE  SEPARATION  OF  MAGNESIUM  FROM  BARIUM, 
STRONTIUM,  AND  CALCIUM 

The  method  is  necessarily  a  combination  of  the  processes  by 
which  magnesium  is  separated  from  the  individual  alkaline 
earths.  The  dilute  solution,  which  is,  of  course,  free  from  sul- 
phuric acid  and  from  phosphoric  or  any  other  acid  which  forms 
with  the  metals  compounds  insoluble  in  neutral  or  alkaline  solu- 
tions, is  treated  with  ammonium  chloride  until  ammonia  gives  no 
precipitate,  then  with  ammonia  to  distinctly  alkaline  reaction, 
and  finally  with  a  slight  excess  of  ammonium  carbonate.  The 
beaker  is  covered  and  allowed  to  stand  in  a  warm  place  for  one 
hour.  The  precipitate  is  collected  upon  a  filter  and  washed  with 
water  to  which  a  little  ammonia  has  been  added. 

The  precipitate  contains,  in  addition  to  the  carbonates  of  the 
alkaline  earths,  a  small  quantity  of  ammonium-magnesium  car- 
bonate. If  the  magnesium  thus  precipitated  cannot  be  neglected, 
the  mixture  of  carbonates,  must  be  dissolved  in  hydrochloric 
acid  and  the  alkaline  earths  reprecipitated  with  sulphuric  acid 
and  alcohol.  The  nitrate  from  the  sulphates  is  evaporated  to 
a  small  volume,  and  the  minute  quantity  of  magnesium  which 


THE  ALKALINE  EARTHS  AND  MAGNESIUM  443 

it  contains  is  precipitated  as  ammonium-magnesium  phosphate. 
The  sulphates  of  the  alkaline  earths  are  converted  into  carbon- 
ates by  fusion  with  potassium-sodium  carbonate,  and  the  metals 
are  separated  and  determined  by  the  methods  of  Fresenius. 

The  filtrate  from  the  carbonates  contains,  besides  the  greater 
portion  of  the  magnesium,  weighable  quantities  of  barium  and 
calcium  —  more  of  the  former  than  of  the  latter.  It  is  treated 
with  3  or  4  drops  of  dilute  sulphuric  acid,  and  afterwards  with 
a  quantity  of  ammonium  oxalate  sufficient  to  convert  all  of  the 
magnesium  into  oxalate.  The  precipitate  contains  the  barium 
as  sulphate,  the  calcium  as  oxalate,  and  it  may  contain  a  small 
amount  of  magnesium  oxalate.  After  standing  for  12  hours 
in  a  moderately  warm  place,  it  is  collected  upon  a  small  filter, 
washed,  and  treated  with  dilute  hydrochloric  acid.  The  barium 
sulphate  remaining  on  the  filter  is  determined  in  the  usual 
manner.  The  filtrate  is  neutralized  with  ammonia  and  treated 
with  a  very  little  ammonium  oxalate.  The  reprecipitated  cal- 
cium oxalate  is  free  from  magnesium.  The  filtrate  is  added  to 
that  from  the  first  precipitation  of  oxalate.  The  solution  con- 
taining the  magnesium  —  except  that  recovered  from  the  car- 
bonates of  the  alkaline  earths  —  is  evaporated  to  dryness  in  a 
platinum  dish,  and  the  ammonium  salts  are  volatilized.  The 
residue  is  dissolved  in  a  little  dilute  hydrochloric  acid  and  the 
magnesium  precipitated  as  ammonium-magnesium  phosphate. 

THE  ANALYSIS  OF  A  SOLUTION  CONTAINING 

THE  ALKALIES,  ALKALINE  EAKTHS, 

AND  MAGNESIUM 

The  separation  and  determination  of  potassium,  sodium,  cal- 
cium, and  magnesium  is  a  frequently  recurring  problem  in 
mineral  analysis.  With  the  calcium  in  minerals  there  is  often 
associated  more  or  less  strontium  and  —  more  rarely  —  a  small 
quantity  of  barium.  The  procedure  here  recommended  provides 
for  the  isolation  and  estimation  of  all  of  the  metals  in  question. 


444  QUANTITATIVE  EXERCISES 

It  may,  of  course,  be  somewhat  simplified  if  one  or  more  of 
them,  e.g.  barium  or  strontium  or  both  barium  and  strontium, 
are  known  to  be  absent. 

1.  The  Preliminary  Treatment.    The  solution,  owing  to  earlier 
treatment  which  it  would  be  premature  to  describe  in  this  place, 
will  be  free  from  phosphoric  acid  and  from  any  other  acid  which 
might,  under  certain  conditions,  precipitate  one  or  more  of  the 
metals.     It  will,  however,  in  consequence  of  the  previous  sepa- 
ration  of  iron,  aluminium,   etc.,  usually  contain   considerable 
quantities  of  ammonium  salts,  which  should  be  removed  before 
proceeding  to  the  separation.     To  this  end,  the  solution  is  evapo- 
rated to  dryness  in  a  platinum  dish  and  the  ammonium  salts  are 
cautiously  volatilized.     The  residue  is  dissolved  in  water  with 
the  aid  of  a  little  hydrochloric  acid,  and  the  solution  consider- 
ably diluted. 

2.  Separation  of  the  Alkaline  Earths  from  Magnesium  and  the 
Alkalies.    The  process  is  essentially  the  same  as  for  the  separa- 
tion of  the  alkaline  earths  from  magnesium.     The  solution  — 
prepared  as  described  under  1  —  will  be  acid.     It  is  treated 
with  ammonia  until  the  reaction  becomes  alkaline.     If  a  precipi- 
tate appears,  hydrochloric  acid  is  cautiously  added  until  it  is 
dissolved,  and  the  solution  is  again  made  alkaline  with  ammo- 
nia.    This  treatment  is  repeated  until  there  is  formed  a  quan- 
tity of  ammonium  chloride  sufficient  to  prevent  the  precipitation 
of  magnesium  by  ammonia.     The  solution  is  then  treated  with 
more  ammonia  and  finally  with  a  slight  excess  of  ammonium 
carbonate.     The  beaker  in  which  the  precipitation  was  made  is 
covered  and  allowed  to  stand  for  an  hour  in  a  warm  place.    The 
precipitate  is  collected  and  washed  with  water  to  which  has 
been  added  a  very  little  ammonia. 

The  precipitate  will  probably  contain  a  small  quantity  of 
ammonium-magnesium  carbonate,  while  the  filtrate  will  con- 
tain some  calcium  and  a  large  quantity  of  barium,  if  that 
element  is  present  in  the  material  under  investigation.  The 
presence  of  strontium  in  the  filtrate  is  hardly  to  be  feared,  owing 


THE   ALKALINE  EARTHS  AND  M AGNES II M  445 

to  the  very  slight  solvent  action  of  ammonium  chloride  on 
strontium  carbonate.  The  precipitate  and  the  nitrate  must, 
therefore,  be  worked  over  —  the  former  for  the  recovery  of 
magnesium  and  the  latter  for  the  recovery  of  calcium,  or  for 
the  recovery  of  both  calcium  and  barium.  The  exact  procedure 
to  be  adopted  will,  in  both  cases,  depend  upon  the  presence  or 
absence  of  other  alkaline  earths  than  calcium. 

The  Precipitate 

If,  as  happens  in  the  majority  of  cases,  calcium  is  the  only 
alkaline  earth  to  be  considered,  the  precipitate  is  dissolved  in 
hydrochloric  acid.  The  dilute  acid  solution  is  made  distinctly 
alkaline  with  ammonia.  If  a  precipitate  appears,  it  is  dissolved 
in  hydrochloric  acid  and  the  solution  is  again  made  alkaline 
with  ammonia.  A  moderate  excess  of  ammonium  oxalate  is 
then  added.  After  12  hours  the  calcium  oxalate  is  collected 
and  washed  in  the  usual  manner.  It  will  probably  be  suffi- 
ciently free  from  magnesium,  but  in  case  exact  results  are 
required,  the  oxalate  may  be  dissolved  in  hydrochloric  acid  and 
the  calcium  reprecipitated  by  the  addition  of  ammonia  and  a 
little  ammonium  oxalate.  The  filtrate  or  filtrates  containing 
the  small  quantity  of  magnesium  recovered  from  the  calcium 
carbonate  are  acidified  with  hydrochloric  acid,  evaporated  to  a 
small  volume,  and  added  to  the  first  filtrate,  which  contains  the 
greater  part  of  the  magnesium. 

If  the  material  under  investigation  contains,  besides  calcium, 
barium  or  strontium,  or  both  barium  and  strontium,  the  pre- 
cipitate of  carbonates  is  dissolved  in  hydrochloric  acid  and  the 
solution  is  treated  with  dilute  sulphuric  acid  and  then  with 
half  its  volume  of  alcohol.  After  standing  for  12  hours  the 
precipitate  is  collected  upon  a  filter  and  washed  with  a  mixture 
of  1  part  of  alcohol  and  2  parts  of  water.  The  filtrate  contains 
the  magnesium  which  was  precipitated  with  the  carbonates  of 
the  alkaline  earths,  and  also  the  portion  of  the  calcium  not 


446  QUANTITATIVE  EXERCISES 

precipitated  by  the  sulphuric  acid.  To  remove  the  latter,  the 
alcohol  is  evaporated  and  the  solution  is  made  alkaline  with 
ammonia  and  treated  with  ammonium  oxalate,  etc.,  as  directed 
above.  By  this  treatment  the  alkaline  earths  will  be  freed  from 
magnesium,  but  they  will  be  left  partly  in  the  form  of  sulphates 
and  partly  in  the  form  of  oxalate.  Both  the  sulphates  and  the 
oxalate  must,  however,  be  converted  into  carbonates  before  the 
metals  can  be  separated.  To  this  end  the  oxalate  is  heated  in 
a  platinum  crucible  to  convert  it  into  carbonate.  The  sulphates 
are  brought  into  the  same  crucible  with  the  calcium  carbonate 
and  mixed  with  sodium-potassium  carbonate.  The  mixture  is 
fused  and  treated  with  water.  The  insoluble  carbonates  are 
collected  upon  a  filter  and  washed  until  they  are  free  from  the 
sulphates  of  the  alkalies.  Finally,  the  alkaline  earths  are  sepa- 
rated and  determined,  as  prescribed  in  Exercise  XLIX.  The 
filtrate  containing  the  small  quantity  of  magnesium  extracted 
from  the  carbonates  of  the  alkaline  earths  should  not,  when 
barium  or  strontium  is  present,  be  incorporated  with  that  which 
contains  the  greater  portion  of  the  magnesium.  It  is  rather  to 
be  evaporated  to  dryness  and  heated  to  expel  ammonium  salts, 
after  which  the  residue  is  to  be  dissolved  and  the  magnesium 
precipitated  as  ammonium-magnesium  phosphate. 

In  order  to  avoid  a  needless  multiplication  of  processes,  the 
isolation  and  determination  of  the  individual  alkaline  earths, 
after  the  removal  of  the  magnesium  precipitated  with  them  by 
ammonium  carbonate,  should  be  postponed  until  the  alkaline 
earths  in  the  filtrate  have  been  recovered. 

The  Filtrate 

The  filtrate  from  the  carbonates  of  the  alkaline  earths  contains 
the  alkalies,  the  magnesium,  —  except  a  small  quantity  which  is 
separately  determined  when  barium  or  strontium  is  associated 
with  the  calcium,  —  some  calcium,  and  possibly  also  barium. 
The  first  step  to  be  taken  is  for  the  recovery  of  the  calcium, 


THE  ALKALINE  EARTHS  AND  MAGNESIUM  447 

or  for  the  recovery  of  calcium  and  barium,  if  the  latter  element 
is  present. 

If  barium  is  absent,  the  solution  will  contain  all  of  the  mag- 
nesium and  some  oxalic  acid,  since  in  that  case  the  magnesium 
recovered  from  the  precipitate  of  calcium  carbonate  is  added 
to  the  nitrate.  It  is  made  alkaline  with  ammonia  and  treated 
with  a  quantity  of  ammonium  oxalate  which  is  certainly  some- 
what more  than  equivalent  not  only  to  the  calcium  but  also  to 
the  magnesium.  The  necessity  for  this  precaution  is  due  to  the 
solubility  of  calcium  oxalate  in  solutions  containing  magnesium 
chloride.  After  12  hours  the  precipitate  of  calcium  oxalate  is 
collected  and  washed  in  the  usual  manner.  It  contains  some 
magnesium  oxalate.  It  is  therefore  dissolved  in  hydrochloric 
acid,  and  the  calcium  is  reprecipitated  by  the  addition  of  am- 
monia and  a  little  ammonium  oxalate.  After  standing  for  12 
hours  the  precipitate,  which  is  now  free  from  magnesium,  is 
collected,  washed,  and  converted  into  carbonate,  either  sepa- 
rately or  with  the  main  body  of  calcium  oxalate.  The  filtrate 
is  added  to  that  which  contains  the  alkalies  and  the  greater 
portion  of  the  magnesium. 

If  the  material  under  examination  contains  barium,  the  pro- 
cedure is  as  follows :  The  nitrate  is  treated  first  with  3  or  4 
drops  of  dilute  sulphuric  acid  and  then  with  ammonium  oxalate 
as  described  above.  After  12  hours  the  precipitate  is  collected, 
washed,  and  treated  on  the  filter  with  hydrochloric  acid.  The 
barium  sulphate  remains  as  an  insoluble  residue,  while  the  cal- 
cium oxalate  and  the  little  magnesium  oxalate  which  was  precipi- 
tated with  it  are  dissolved.  The  solution  is  made  alkaline  with 
ammonia  and  treated  with  a  small  quantity  of  ammonium  oxalate. 
The  reprecipitated  calcium  oxalate  is  free  from  magnesium,  and 
the  filtrate  from  it  is  added  to  that  which  contains  the  alkalies 
and  the  greater  portion  of  the  magnesium.  The  barium  and  the 
calcium  thus  recovered — the  former  as  sulphate  and  the  latter 
as  oxalate  —  can,  of  course,  be  dealt  with  separately,  or  be  added 
to  and  worked  over  with  the  main  body  of  the  alkaline  earths. 


448  QUANTITATIVE  EXERCISES 

3.  Separation  of  Magnesium  from  the  Alkalies.  After  the 
removal  of  the  alkaline  earths  in  the  manner  previously 
described,  the  solution  contains  the  alkalies,  nearly  all  of  the 
magnesium,  —  all  of  it  when  calcium  is  the  only  alkaline  earth 
to  be  determined,  —  and  considerable  quantities  of  ammonium 
salts.  The  acids  which  are  represented  in  the  solutions  by 
their  salts  are  hydrochloric,  oxalic,  and  sulphuric.  The  last 
of  these,  sulphuric  acid,  can  be  present  only  when  barium  is 
one  of  the  constituents  to  be  estimated,  and  even  then  only 
in  small  quantity.  The  first  step  to  be  taken  has  for  its  object 
the  removal  of  the  ammonium  salts  in  such  a  manner  as  to  leave 
the  alkalies  and  the  magnesium  in  the  form  of  chlorides.  The 
solution  is  acidified  with  hydrochloric  acid  and  evaporated  to 
dryness  in  a  platinum  dish.  The  residue  is  then  cautiously 
ignited  to  expel  the  ammonium  compounds.  Owing  to  the 
presence  of  oxalates,  a  portion  of  the  alkalies  may,  during  the 
ignition,  be  converted  into  carbonates ;  hence  the  material  is 
repeatedly  treated  with  small  quantities  of  ammonium  chloride 
and  reheated.  This  course  will  also  make  more  certain  the 
removal  of  the  last  traces  of  sulphuric  acid.  The  chlorides  are 
dissolved  in  water  and  the  magnesium  is  separated  and  deter- 
mined as  oxide  by  the  mercuric  oxide  method.  Finally,  the 
alkalies  are  isolated  and  weighed  as  chlorides  and  afterwards 
separated  by  means  of  platinum  chloride. 

THE   HAKDNESS   OF   WATEE 

The  so-called  "  hardness  "  of  water  is  due  to  the  presence  in 
it  of  soluble  salts  of  calcium  and  magnesium.  These  react  with 
the  oleates,  palmitates,  and  stearates  of  the  alkalies,  of  which 
soaps  are  largely  made  up,  forming  insoluble  oleates,  palmitates, 
and  stearates  of  calcium  and  magnesium.  Hence  the  proper 
action  of  soap  upon  fatty  substances  cannot  be  secured  until 
all  of  the  calcium  and  magnesium  in  a  water  have  been  precip- 
itated. The  soluble  salts  of  barium  and  strontium  behave 


THE  ALKALINE  EARTHS  AND  MAGNESIUM  449 

towards  soap  in  the  same  manner  as  those  of  calcium  and  mag- 
nesium ;  the  fact  is,  however,  of  little  significance,  owing  to  the 
very  rare  occurrence  of  these  metals  in  natural  waters. 

The  hardness  of  water  is  of  two  kinds,  —  "temporary"  (that 
which  disappears  when  the  water  is  boiled)  and  "permanent" 
(that  which  is  not  affected  by  boiling).  The  temporary  hard- 
ness is  due  to  the  presence  of  the  acid  carbonates  of  calcium 
and  magnesium  which  are  decomposed  by  heat  with  deposition  of 
the  neutral  insoluble  carbonates,  while  the  permanent  hardness 
is  caused  by  those  salts,  mainly  chlorides  and  sulphates,  whose 
composition  is  not  changed  in  boiling  aqueous  solutions. 

It  is  customary  to  estimate  the  hardness  of  a  water  by  means 
of  a  standard  alcoholic  solution  of  soap  rather  than  by  a  quan- 
titative determination  of  the  calcium  and  magnesium  which  it 
contains.  The  soap  solution  is  standardized  by  means  of  a 
solution  of  calcium  chloride  of  known  concentration. 

There  is  as  yet  no  satisfactory  uniformity  of  practice  as 
regards  the  statement  of  results.  In  other  words,  the  so-called 
"  degree  "  of  hardness  has  a  different  significance  in  different 
countries  and  sometimes  within  the  limits  of  the  same  country. 
In  France,  and  usually  in  England  and  America,  a  milligram 
of  calcium  carbonate  and  100  cc.  of  water  are  made  the  basis  of 
computation ;  while  in  Germany  the  same  weight  of  calcium 
oxide  and  an  equal  volume  of  water  are  made  the  units.  In  the 
former  countries  a  water  is  said  to  have  1,  2,  3,  etc.,  degrees  of 
hardness  when  100  cc.  of  it  will  destroy  as  much  soap  as  1,  2, 
3,  etc.,  milligrams  of  calcium  carbonate  dissolved  in  the  same 
volume  of  water.  The  German  "  degrees  of  hardness  "  are,  of 
course,  related  to  those  of  the  first-named  countries  inversely  as 
the  molecular  weights  of  CaO  and  CaCO3,  or  directly  as  99.34 
to  55.68.  In  England  and  America  there  are  also  used,  to  some 
extent,  the  gallon  as  the  unit  measure  of  the  water,  and  the 
grain  as  the  unit  weight  of  the  calcium  carbonate.  The  Eng- 
lish or  "imperial"  gallon  of  water  weighs  70,000  grains,  while 
100  cc.  of  water  weighs  100,000  milligrams;  hence  degrees  of 


450  QUANTITATIVE  EXERCISES 

hardness  calculated  on  the  basis  of  the  imperial  gallon  and  the 
grain  are  related  to  those  calculated  by  the  usual  standard  as 
10  to  7.  The  United  States  gallon  of  water  weighs  approxi- 
mately 58319  grains ;  hence  degrees  of  hardness  calculated  on 
the  basis  of  the  U.  S.  gallon  and  the  grain  are  related  to  those 
calculated  by  the  usual  standards  as  10  to  5.8319.  The  follow- 
ing table  gives  the  numbers  by  which  degrees  of  hardness  of 
any  one  kind  are  to  be  multiplied  in  order  to  convert  them  into 
the  equivalent  number  of  degrees  of  other  kinds. 

To  Convert 

Degrees  French,  Eng.,  and  Am.  into  degrees  German      .     .  0.560 

Degrees  German  into  degrees  French,  Eng.,  and  Am.      .     .  1.786 

Grains  per  imperial  gallon  into  milligrams  per  100  cc.     .     .  1.429 

Milligrams  per  100  cc.  into  grains  per  imperial  gallon      .     .  0.700 

Grains  per  U.S.  gallon  into  milligrams  per  100  cc.      ...  1.715 

Milligrams  per  100  cc.  into  grains  per  U.S.  gallon  ....  0.583 

Grains  per  imperial  gallon  into  grains  per  U.S.  gallon     .     .  0.833 

Grains  per  U.S.  gallon  into  grains  per  imperial  gallon     .     .  1.200 


EXERCISE  L 
DETERMINATION  OF  THE   HARDNESS  OF  WATER 

There  are  required : 

1.  A  standard  solution  of  calcium  chloride.     Weigh  into  a 
platinum  or  porcelain  dish  1  gram  of  pure  calcium  carbonate. 
Treat  with  hydrochloric  acid  and  cover  the  dish  with  a  watch 
glass.     When  the  material  is  dissolved,  wash  the  under  side  of 
the  cover  back  into  the  dish  and  evaporate  the  solution  to  dry- 
ness.     Moisten  the  residue  with  distilled  water  and  again  evap- 
orate, repeating  the  operation  until  the  excess  of  the  hydrochloric 
acid  has  been  expelled.     Dissolve  the  neutral  residue  in  water 
and  dilute  to  1  liter. 

2.  A  standard  solution  of  soap.     Rub  together  in  a  porcelain 
mortar  15  parts  of  lead  plaster  (emplastrum  plumbi)  and  4  parts 
by  weight  of  pure  dry  potassium  carbonate  until  a  homogeneous 


THE  ALKALINE  EARTHS  AND  MAGNESIUM  451 

mass  is  obtained.  Add  strong  alcohol,  mix  well,  and  then  allow 
the  insoluble  matter  to  settle.  Filter,  and  wash  the  lead  car- 
bonate with  alcohol.  Let  the  filtrate  stand  for  a  few  hours, 
filter  again  if  necessary,  distill  off  the  alcohol,  and  dry  the  soap 
residue  upon  the  water  bath. 

Dissolve  10  grams  of  the  soap  in  half  a  liter  of  56  per  cent 
alcohol,  and  proceed  to  standardize  the  solution  in  the  following 
manner. 

Measure  10  cc.  of  the  standard  calcium  chloride  solution  into 
a  glass-stoppered  bottle  or  cylinder  having  a  capacity  of  200  or 
250  cc.,  and  dilute  with  recently  boiled  distilled  water  to  a 
volume  of  50  cc.  Add  the  soap  solution  from  a  burette,  1  cc. 
at  a  time,  shaking  vigorously  after  each  addition  until  evidences 
of  a  tendency  to  form  a  lather  are  observed.  Afterwards  pro- 
ceed more  cautiously,  adding  smaller  quantities  of  the  soap  and 
placing  the  vessel  on  its  side  after  agitating  its  contents  until  a 
lather  is  obtained  which  persists  not  less  than  5  minutes.  Repeat 
the  experiment  until  a  series  of  concordant  results  have  been 
obtained,  taking  care,  however,  never  to  add  more  than  1  cc.  of 
the  soap  solution  at  a  time,  even  though  the  total  volume  which 
will  be  required  is  approximately  known. 

Having  found  the  relation  of  the  soap  solution  to  that  of  the 
calcium  chloride,  dilute  the  former  by  stages  with  a  mixture  of 
1  part  of  water  and  2  parts  of  alcohol  until  14.25  cc.  of  the  soap  are 
exactly  equivalent  to  10  cc.  of  the  calcium  chloride  solution  when 
the  latter  is  diluted  with  distilled  water  to  a  volume  of  50  cc. 

In  the  place  of  the  soap  prepared  as  directed  above,  a  good 
quality  of  Castile  or  of  shaving  soap  may  be  used  whenever  the 
stability  of  the  standard  solution  is  of  no  consequence. 

1.  DETERMINATION  OF  TOTAL  HARDNESS 

Measure  50  cc.  of  tap  water  into  the  vessel  used  in  standard- 
izing the  soap  solution.  Agitate  for  a  few  moments  and  then 
suck  out  the  air  above  the  water  through  a  glass  tube.  Titrate 


452  QUANTITATIVE  EXERCISES 

with  the  soap  solution  in  exactly  the  same  manner  as  when 
determining  its  strength  by  means  of  the  standard  solution  of 
calcium  chloride.  The  hardness  of  the  water  corresponding  to 
the  volume  of  the  soap  consumed  will  be  found  in  the  accom- 
panying table. 

If  more  than  16  cc.  of  the  soap  solution  are  required  to  pro- 
duce a  lather  of  the  requisite  permanence  in  50  cc.  of  a  water, 
a  measured  portion  of  the  latter,  e.g.  25  cc.,  should  be  diluted 
with  distilled  water  to  50  cc.  and  the  determination  repeated. 
The  degree  of  hardness  as  found  in  the  table  is  then,  of 
course,  to  be  multiplied  by  a  number  corresponding  to  the 
dilution. 

If  the  hardness  of  a  water  is  largely  due  to  magnesium  salts, 
—  a  condition  which  is  made  apparent  by  the  characteristic  light- 
ness of  the  curd,  —  the  water  should  be  diluted  until  not  more 
than  7  cc.  of  the  soap  are  required  to  produce  a  lather.  Such 
waters,  moreover,  should  be  tested,  after  having  stood  for  half 
an  hour  or  longer,  with  reference  to  their  power  to  reproduce 
the  lather  when  agitated. 


2.  DETERMINATION  OF  PERMANENT  HARDNESS 

Gently  boil  a  measured  quantity  of  tap  water  in  an  open 
vessel,  and  when  it  has  cooled,  dilute  to  the  original  volume 
with  recently  boiled  distilled  water.  Determine  the  hardness 
in  the  same  manner  as  before. 

The  temporary  hardness  is  the  difference  between  the  total 
and  the  permanent  hardness. 


THE  ALKALINE  EARTHS  AND  MAGNESIUM 


453 


Table  of  Hardness,  or  Parts  of  OaC03  per  100,000  Parts  of 
Water.      Quantity  of  Water  used,  50  cc. 


*i 

£§• 

A 

sl 

Cc.  of 
Soap  Sol. 

A 

On 
I 

ol 

6| 

P 

h 

^02 

o9< 
O  0 

u  ® 

°! 

*i 
si 

03 

O  cT 
oo 

al 

SH'O 
005 

0  A 

^  c8 

A 

Oo 

8j 

.7 

.00 

3.3 

3.64 

5.9 

7.29 

8.5 

11.05 

11.1 

15.00 

13.7 

19.13 

.8 

.16 

.4 

.77 

6.0 

.43 

.6 

.20 

.2 

.16 

.8 

.29 

.9 

.32 

.5 

.90 

.1 

.57 

.7 

.35 

.3 

.32 

.9 

.44 

1.0 

.48 

.6 

4.03 

.2 

.71 

.8 

.50 

.4 

.48 

14.0 

.60 

.1 

.63 

.7 

.16 

.3 

.86 

.9 

.65 

.5 

.63 

.1 

.76 

.2 

.79 

.8 

.29 

.4 

8.00 

9.0 

.80 

.6 

.79 

.2 

.92 

.3 

.95 

.9 

.43 

.5 

.14 

.1 

.95 

.7 

.95 

.3 

20.08 

.4 

1.11 

4.0 

.57 

.6 

.29 

.2 

12.11 

.8 

16.11 

.4 

.24 

.5 

.27 

.1 

.71 

.7 

.43 

.3 

.26 

.9 

.27 

.5 

.40 

.6 

.43 

.2 

.86 

.8 

.57 

.4 

.41 

12.0 

.43 

.6 

.56 

.7 

.56 

.3 

5.00 

.9 

.71 

.5 

.56 

.1 

.59 

.7 

.71 

.8 

.69 

.4 

.14 

7.0 

.86 

.6 

.71 

.2 

.75 

.8 

.87 

.9 

.82 

.5 

.29 

.1 

9.00 

.7 

.86 

.3 

.90 

.9 

21.03 

2.0 

.95 

.6 

.43 

.2 

.14 

.8 

13.01 

.4 

17.06 

15.0 

.19 

.1 

2.08 

.7 

.57 

.3 

.29 

.9 

.16 

.5 

.22 

.1 

.35 

.2 

.21 

.8 

.71 

.4 

.43 

10.0 

.31 

.6 

.38 

.2 

.51 

.3 

.34 

.9 

.86 

.5 

.57 

.1 

.46 

.7 

.54 

.3 

.68 

.4 

.47 

5.0 

6.00 

.6 

.71 

.2 

.61 

.8 

.70 

.4 

.85 

.5 

.60 

.1 

.14 

.7 

.86 

.3 

.76 

.9 

.86 

.5 

22.02 

.6 

.73 

.2 

.29 

.8 

10.00 

.4 

.91 

13.0 

18.02 

.6 

.18 

.7 

.86 

.3 

.43 

.9 

.15 

.5 

14.06 

.1 

.17 

.7 

.35 

.8 

.99 

.4 

.57 

8.0 

.30 

.6 

.21 

.2 

.33 

.8 

.52 

.9 

3.12 

.5 

.71 

.1 

.45 

.7 

.37 

.3 

.49 

.9 

.69 

3.0 

.25 

.6 

.86 

.2 

.60 

.8 

.52 

.4 

.65 

16.0 

.86 

.1 

.38 

.7 

7.00 

.3 

.75 

.9 

.68 

.5 

.81 

.2 

.51 

.8 

.14 

.4 

.90 

11.0 

.84 

.6 

.97 

CHAPTER  XX 
POTASSIUM  PERMANGANATE 

In  the  presence  of  oxidizable  substances  potassium  perman- 
ganate is  reduced.  If  the  solutions  to  which  it  is  added  are 
acid,  the  reduction  is  generally  complete  in  accordance  with  the 
following  equation : 

2  KMn04  +  3  H2SO4  =  K2SO4  +  2  MnSO4  +  3  H2O  +  5  O. 

In  other  words,  each  molecule  of  potassium  permanganate  in 
acid  solutions  usually  yields  2£  atoms  of  oxygen  which  are 
available  for  oxidizing  purposes.  If,  on  the  other  hand,  the 
solutions  are  neutral  or  alkaline,  the  reduction  is  incomplete 
and  not  always  uniform  in  extent.  The  manganese  of  th.e  per- 
manganate is,  in  such  cases,  precipitated  in  the  form  of  an  oxide 
which  contains  relatively  more  oxygen'  than  MnO  and  which 
may  have  the  composition  represented  by  MnO2. 

The  following  equations  represent  some  of  the  more  important 
of  those  reactions  of  potassium  permanganate  which  are  employed 
for  the  quantitative  determination  of  oxidizable  substances. 

1.  2  KMn04  +  5  H2C3O4  2  H2O  +  3  H2SO4 

=  K2SO4  +  2  MnSO4  +  18  H2O  +  10  C02. 
1  molecule  KMnO4  =  2£  molecules  of  oxalic  acid. 
1  molecule  KMnO4  =  2£  molecules  of  an  acid  oxalate. 
1  molecule  KMnO4  =  1?  molecules  of  potassium  tetroxalate. 

2.  2  KMnO4  +  10  FeSO4  +  8  H2SO4 

-  K2S04  +  2  MnS04  +  5  Fe23  SO4  +  8  H2O. 
1  molecule  KMnO4  =  5  atoms  of  ferrous  iron. 

3.  2  KMn04  +  5  H2O2  +  3  H2SO4 

=  K2SO4  +  2  MnSO4  4-  8  H2O  +  5  O2. 
1  molecule  KMnO4  =  2|  molecules  of  H2O2. 
454 


POTASSIUM  PERMANGANATE  455 

4.  2  KMnO4  +  5  HNO2  +  3  H2SO4 

=  K2SO4  +  2  MnSO4  +  5  HNO3  +  3  H2O. 
1  molecule  KMnO4  =  2£  molecules  HNO2  or  1£  molecules  N2O8. 

5.  2  KMnO4  +  3  MnSO4  +  2  H2O 

=  2  HKSO4  +  H2SO4  +  5  MnO2. 
1  molecule  KMnO4  =  1£  atoms  of  manganese. 

Atomic  Weights 

Oxygen 15.88  Sulphur 31.83 

Carbon 11.9  Nitrogen 13.93 

Potassium 38.82  Manganese 54.6 

Iron  55.5 


Molecular  Weights  and  Equivalent  Numbers 

Potassium  permanganate    ....  156.94    Equivalent  number    156.94 

Oxalic  acid 125.08  «  «  312.7 

Acid  potassium  oxalate 127.14  "  «  317.85 

Potassium  tetroxalate 252.22  "  «  315.28 

Ferrous  ammonium  sulphate .     .     .  389.34  "  •    «  1946.7 

Hydrogen  peroxide 33.76  "  "  84.4 

Nitrous  acid  anhydride 75.5  "  "  94.38 

Ferrous  oxide 71.38  «  «  356.9 

Metallic  iron 55.5  «  "  £77.5 

Ferric  oxide 158.64  «  «  396.6 

Manganese 54.6  «  «  81.9 

Manganous  oxide 70.48  «  «  105.72 

The  weight  of  1  cc.  of  oxygen,  latitude  45°,  sea  level,  temperature  0°, 
pressure  760  mm.,  is  0.001429908  gram. 

The  table  on  page  457  is  designed  to  facilitate  the  computa- 
tion of  analytical  results.  It  contains  the  substances  more  com- 
monly determined  by  potassium  permanganate,  and  gives,  for 
any  unit  quantity  of  each,  the  equivalent  quantities  of  the 
others. 

Three  examples  will  sufficiently  illustrate  the  method  of 
using  the  table. 


456  QUANTITATIVE  EXERCISES 

1.  Suppose  it  is  required  to  find  how  much  potassium  tetroxa- 
late  is  equivalent,  in  reducing  power,  to   100  milligrams  of 
ferrous  oxide.     The  salt  will  be  found  to  be  No.  IV  in  the 
column  giving  the  composition  of  the  substances  ;  and  by  follow- 
ing column  IV  down  to  the  horizontal  line  on  which  the  equiva- 
lents of  ferrous  oxide  are  given,  the  number  0.88338  will  be 
found.     This,  multiplied  by  the  weight  of  the  ferrous  oxide, 
will  give  the  equivalent  weight  of  potassium  tetroxalate,  i.e. 
88.338  milligrams. 

2.  Suppose  it  is  required  to  find  how  much  ferrous  oxide 
100  cc.  of  oxygen,  under  standard  conditions,  will  convert  into 
ferric  oxide.     By  following  column  VI  down  to  the  line  of 
oxygen  volume-equivalents,  the  number  0.0128473149  will  be 
found,  and  this  multiplied  by  100   will  give  the   weight  of 
ferrous  oxide  which  100  cc.  of  oxygen  will  convert  into  ferric 
oxide,  i.e.  1.28473  grams. 

3.  In  the  same  way,  by  following  down  columns  XIII  and 
XIV  to  VI,  11.214  milligrams  and  7.784  cc.  would  be  found 
to  be  the  weight  and  volume  respectively  of  the  oxygen  re- 
quired to  convert  100.  milligrams  of  FeO  into  Fe2O3. 

The  substances  which  are  ordinarily  employed  to  determine 
the  strength  of  potassium  permanganate  solutions  are  oxalic 
acid,  acid  potassium  oxalate,  potassium  tetroxalate,  ferrous 
ammonium  sulphate,  and  metallic  iron  in  the  form  of  wire. 
Other  substances  not  so  frequently  used  for  this  purpose  are 
sodium  oxalate,  lead  oxalate,  ferric  ammonium  sulphate  reduced 
by  metallic  zinc,  manganous  sulphate,  and  acidified  solutions  of 
potassium  iodide  from  which  potassium  permanganate  liberates 
an  equivalent  quantity  of  iodine. 


1.   Oxalic  Acid 

The  preparation  of  oxalic  acid  entirely  free  from  acid  oxalates 
of  the  alkalies  is  a  matter  of  some  difficulty.  Simple  recrystal- 
lization  from  cooling  saturated  solutions  does  not  suffice  owing 


POTASSIUM  PERMANGANATE 


457 


C  •  ti  5    tf  •  8 ,'  i:    3 


d     d     d       o'       o"     o       d       d     o'     d 


i  §  I  § 

*    £5    o    8 

o"     d     i-i     d 


S    8    3 

i  I  I 


odd       d       do       o"       d     r4     ,-i     i-°     »-* 


CD        1-1        t-          t>          rf        CO 
CO        O        CO          t~          I-H        ^ 

odd       o"       d     d 


d       d     o     d     !-«'     d 


d     d       d       do       o"       d     r-i     r-J     rn     d 


o"     d     d       d       d     d       d       d     »-H     d     i-i     d 


TH        CO 

CO        ^ 
M        <M 


rH         ^H  O  »H 


(M        IO        CO  ^H  5        O 


to"     co     d 


O          iHO  O          OCOdCOCNCDO 


o       ococoeocit-o 


co      co      co      co      o 


II    111 

»-J     d       d       »H     d 


d       d 


ss.   q 

t-        O 


THOO  O  00  O  Oi-li-Hrli-ICOO 


iifl^ 


'    "    "  f  I 

O     ^    #       *     O      f"  1 

e  g  &  I  I  =-  5 


458  QUANTITATIVE  EXERCISES 

to  the  difficult  solubility  of  such  oxalates  in  cold  water  and 
their  ready  solubility  in  hot  water.  Since,  however,  the  molec- 
ular weights  of  the  acid  oxalates  of  the  alkalies  do  not  differ 
greatly  from  the  molecular  weight  of  oxalic  acid  with  its  two 
molecules  of  water  of  crystallization,  the  entire  removal  of  the 
former  is  not  necessary.  An  acid  sufficiently  pure  for  the 
standardization  of  potassium  permanganate  may  be  prepared 
by  saturating  cold  water  with  the  commercial  acid  and  evapo- 
rating to  crystallization.  This  process,  two  or  three  times 
repeated,  gives  a  product  which  yields  but  little  residue  on 
ignition.  A  still  purer  acid  is  obtained  by  recrystallizing,  first 
from  hot  dilute  hydrocholoric  acid  (10  to  15  per  cent),  then 
from  hot  alcohol,  and  finally  from  water.  The  aqueous  solution 
must  be  heated  until  the  odor  of  ethyl  oxalate  disappears.  The 
purest  oxalic  acid  is  prepared  by  dissolving  in  a  mixture  of 
equal  parts  of  alcohol  (95  per  cent)  and  ether.  The  solution  is 
filtered,  diluted  with  water,  freed  from  the  ether  and  a  portion 
of  the  alcohol  by  distillation,  and  then  evaporated  on  the  water 
bath  to  the  crystallizing  point.  If  the  acid  thus  obtained  has 
the  odor  of  ethyl  oxalate,  it  must  be  again  dissolved  in  water 
and  recrystallized. 

Oxalic  acid  is  stable  at  ordinary  temperatures  in  an  atmos- 
phere containing  the  usual  amount  of  moisture.  It  cannot, 
however,  be  dried,  either  in  a  desiccator  or  in  a  hot-air  bath. 
At  ordinary  temperatures  potassium  permanganate  —  even  in 
the  presence  of  sulphuric  acid  —  acts  very  slowly  upon  oxalic 
acid.  The  presence  of  manganous  salts  greatly  facilitates  the 
reaction.  In  practice  it  is  customary  to  hasten  the  reduction 
of  the  permanganate  by  heating  the  solution  of  oxalic  acid  to 
60°  or  65°.  Once  well  started,  the  reaction  —  owing  to  the 
accumulation  of  manganous  sulphate  in  the  solution  —  proceeds 
rapidly  even  though  the  temperature  of  the  solution  falls  nearly 
to  that  of  the  surrounding  air. 


POTASSIUM  PERMANGANATE  459 

2.   Potassium  Tetroxalate 

This  salt  is  usually  preferred  to  oxalic  acid  for  the  standard- 
ization of  potassium  permanganate.  Its  preparation  has  already 
been  described.  Like  oxalic  acid,  it  is  stable  in  the  air  at  ordi- 
nary temperatures,  but  cannot  be  dried  in  a  desiccator  or  in  a 
hot-air  bath. 

3.  Ferrous  Ammonium  Sulphate 

The  salt  is  dissolved  in  cold  water  and  precipitated  by  alcohol. 
The  precipitate,  which  should  be  entirely  free  from  any  yellow 
color,  is  collected  upon  a  filter  and  washed  with  alcohol.  Fer- 
rous ammonium  sulphate  suffers  some  oxidation  in  contact  with 
the  air,  and  should,  therefore,  be  freshly  prepared. 


EXERCISE  LI 

DETERMINATION  OF  IRON  BY  MEANS  OF  POTASSIUM 
PERMANGANATE 

I.   PREPARATION  OF  THE  PERMANGANATE  SOLUTION 

Dissolve  about  3.3  grams  of  potassium  permanganate  in  water. 
Filter  the  solution  through  asbestus  and  dilute  the  filtrate  to 
1  liter.  Weigh  into  beaker  glasses  two  portions  (from  0.100  to 
0.150  gram  each)  of  pure  potassium  tetroxalate.  Dissolve  in 
water,  heat  to  about  60°,  add  2  or  3  cc.  of  dilute  sulphuric  acid, 
and  titrate  to  a  barely  perceptible  rose  color  with  the  perman- 
ganate solution.  Calculate  from  the  results  the  amount  of  per- 
manganate in  each  cubic  centimeter  of  the  solution.  Divide 
2.82775,  the  number  of  grams  required  for  1  liter,  by  the  quantity 
contained  in  1  cc.,  and  thus  find  how  many  cubic  centimeters 
must  be  diluted  to  a  liter.  Each  cubic  centimeter  of  a  solu- 
tion of  permanganate  prepared  in  this  manner  is  equivalent  to 
5  milligrams  of  iron. 


460  QUANTITATIVE   EXERCISES 

Verify  the  strength  of  the  standard  solution  by  titrating  it 
against  weighed  quantities  of  potassium  tetroxalate  and  also  of 
freshly  prepared  ferrous  ammonium  sulphate.  Finally,  test  the 
solution  again  in  the  following  manner  :  Measure  out  25  cc. 
of  the  permanganate  solution,  add  a  measured  quantity  of  a 
standard  solution  of  sulphuric  acid  which  will  more  than  suffice 
to  neutralize  the  metals  in  the  permanganate,  and  then  reduce 
with  the  least  possible  amount  of  neutral  hydrogen  superoxide. 
Determine  the  excess  of  the  sulphuric  acid  with  a  dilute  stand- 
ard solution  of  ammonia,  using  litmus  as  the  indicator.  The 
reaction  involves  the  neutralization  of  1J  molecules  of  sulphuric 
acid  for  each  molecule  of  potassium  permanganate.  Commercial 
hydrogen  superoxide  may  be  brought  to  a  neutral  condition  by 
agitating  it  with  ignited  zinc  oxide  and  then  filtering  through 
asbestus. 

It  will  be  seen  that  having  a  solution  of  potassium  permanga- 
nate of  known  strength,  one  may  readily  standardize  a  solution 
of  sulphuric  acid  by  the  same  method. 

Solutions  of  permanganate  are  not  wholly  stable.  The  usual 
cause  of  their  deterioration  is  the  presence  in  them  of  the  oxides  of 
manganese.  The  salt  in  solution  is  decomposed  by  these  oxides, 
which  are  often  mingled  with  it  in  the  dry  state,  or  which  form 
when  the  solutions  are  allowed  to  stand  for  some  time.  The 
decomposition  is  attended  by  the  evolution  of  oxygen,  and  a 
small  quantity  of  the  oxide  will  decompose  an  unlimited  amount 
of  the  permanganate,  though  the  rate  of  the  decomposition  in- 
creases rapidly  with  the  quantity  of  the  oxide.  The  rate  of  the 
decomposition  is  also  affected  by  the  temperature  and  by  the 
light.  All  solutions  of  permanganate  should  therefore  be  filtered 
through  asbestus  when  made,  and  those  which  have  been  made 
for  some  time  should  be  refiltered  into  clean  bottles.  Moreover, 
a  solution  which  has  been  exposed  in  a  burette  should  be  filtered 
before  returning  it  to  the  bottle  from  which  it  was  taken. 

In  all  quantitative  determinations  which  involve  a  complete 
reduction  of  potassium  permanganate,  the  solutions  of  the 


1'OTASSH  M    PERMANGANATE  461 

substances  to  be  determined  must  be  sufficiently  acidified  with 
sulphuric  acid.  If  an  insufficient  quantity  of  the  acid  is  used, 
the  solutions  become  brown  and  turbid  in  consequence  of  the 
precipitation  of  manganese  oxides,  and  the  reduction  is  incom- 
plete. The  slight  rose  color  which  is  imparted  to  the  solution 
by  a  small  excess  of  permanganate,  and  which  is  relied  upon  to 
indicate  the  completion  of  the  reaction,  soon  disappears  owing 
to  the  action  of  permanganate  on  manganous  salts.  This  oxi- 
dation of  the  manganese  in  manganous  salts  is  slow,  however, 
under  ordinary  conditions,  and  does  not  take  place  in  the  pres- 
ence of  more  easily  oxidized  matter ;  hence  manganous  salts  do 
not  interfere  with  the  use  of  permanganate  in  the  determination 
of  iron,  oxalic  acid,  etc. 

The  presence  of  hydrochloric  acid  and  of  chlorides  is  to  be 
avoided  if  possible.  They  reduce  the  permanganate  with  liber- 
ation of  chlorine,  but  the  irregularities  due  to  this  reaction  may 
be  avoided  by  adding  manganous  sulphate  to  the  solution.  The 
difficulty  may  also  be  overcome  to  a  great  extent  by  largely 
diluting  the  solution  of  the  substance  to  be  determined. 

II.    DETERMINATION  OF  METALLIC  IRON 

The  "  Bunsen  valve  "  which  is  required  for  this  experiment 
is  made  by  slipping  a  piece  of  small,  black,  rather  thick  rubber 
tubing,  50  or  60  mm.  long,  over  a  round  wooden  rod,  and  cut- 
ting in  the  middle  of  it  a  short  longitudinal  slit.  The  rod 
should  not  be  so  large  as  to  stretch  the  rubber  to  any  great 
extent,  and  care  should  be  taken  that  the  tubing  is  not  twisted 
on  the  rod  when  the  slit  is  made.  The  knife  should  be  wet  and 
very  sharp.  One  end  of  the  tube  is  closed  with  a  short  glass 
rod,  and  the  other  is  slipped  over  the  end  of  a  glass  tube.  The 
other  end  of  the  glass  tube  is  passed  through  a  perforated 
rubber  stopper  which  fits  a  small  flask  in  which  the  iron  is 
to  be  dissolved.  Neither  the  glass  rod  nor  the  tube  should 
be  pushed  so  far  into  the  rubber  as  to  stretch  it  in  the  vicinity 


462  QUANTITATIVE   EXERCISES 

of  the  slit.  Such  an  arrangement,  if  properly  made,  permits 
the  gas  which  is  generated  within  the  flask  to  escape  freely 
into  the  air,  but  closes  effectually  against  any  current  in  the 
other  direction. 

Dissolve  from  100  to  150  milligrams  of  carefully  cleansed 
piano  wire  in  an  excess  of  quite  dilute  sulphuric  acid.  The 
flask  and  its  contents  may  be  moderately  heated  to  expedite  the 
reaction.  When  the  iron  has  dissolved,  remove  the  flask  from 
the  bath  and  introduce  a  very  small  quantity  of  acid  sodium 
carbonate  which  is  free  from  iron.  When  the  solution  has 
cooled  to  50°  or  60°,  open  the  flask  again  and  introduce  an- 
other small  quantity  of  the  carbonate.  Titrate  to  color  as  soon 
as  the  evolution  of  carbon  dioxide  ceases. 

If  the  specimen  of  wire  is  one  which  deposits  carbon  when 
dissolved  in  sulphuric  acid,  the  experiment  is  better  carried  out 
in  the  following  manner :  The  iron  and  the  sulphuric  acid  are 
placed  in  a  quarter-liter  measuring  flask  which  is  connected  with 
another  small  flask  by  means  of  a  glass  tube  bent  to  two  right 
angles.  The  tube  is  cut  in  the  middle  of  the  horizontal  portion 
and  again  joined  by  means  of  a  short  piece  of  rubber  tubing 
which  may  be  closed  by  a  Mohr  pinchcock.  The  end  towards 
the  measuring  flask  passes  'through  a  rubber  stopper  and  reaches 
to  a  point  just  above  the  mark  on  the  neck,  while  the  other  end 
reaches  nearly  to  the  bottom  of  the  second  flask,  in  which  has 
been  placed  a  quantity  of  boiled  distilled  water.  When  the 
iron  is  dissolved  the  rubber  tube  is  closed  and  the  contents  of 
the  flasks  are  allowed  to  cool  to  the  temperature  of  the  air. 
The  measuring  flask  is  then  filled  to  the  mark  by  opening  the 
pinchcock,  and  the  contents  are  well  shaken.  Finally,  when 
the  carbon  has  subsided,  measured  portions  of  the  liquid  are 
withdrawn  and  titrated  with  permanganate. 


POTASSIUM  PERMANGANATE  463 

III.    CONFIRMATION  OF  THE  RESULTS  BY  THE  GRAVIMETRIC 

METHOD 

Dissolve  another  portion  of  the  wire  (from  0.100  to  0.200  gram) 
in  nitric  acid  in  a  covered  porcelain  dish.  Wash  anything  which 
may  have  spattered  upon  the  under  side  of  the  cover  back  into  the 
dish.  Evaporate  the  solution  upon  a  water  bath  and  moisten 
the  residue  with  strong  nitric  acid.  Evaporate  again  and  heat 
the  residue  in  a  hot-air  bath  for  an  hour  at  120°.  Moisten  the 
dried  material  with  a  few  drops  of  strong  nitric  acid,  add  water, 
and  heat  for  half  an  hour  on  the  water  bath.  Filter,  and  wash 
the  paper.  Heat  the  nitrate  and  washings  nearly  to  the  boiling 
point  and  precipitate  the  iron  with  a  small  excess  of  ammonia. 
Add  the  ammonia  slowly  and  stir  the  liquid  constantly  and  vig- 
orously during  precipitation.  Keep  the  vessel  containing  the 
precipitate  hot  for  an  hour  or  longer,  and  then  set  it  aside  for 
several  hours.  Filter,  wash  the  precipitate  with  hot  water,  and 
partially  dry  the  paper.  While  the  paper  is  still  somewhat  moist 
roll  it  together  and  place  it  in  a  weighed  porcelain  crucible.  H  eat 
the  crucible  very  cautiously  until  the  paper  is  dry  and  begins  to 
blacken.  Burn  it  slowly  and  then  moisten  the  mass  with  two  ©r 
three  drops  of  strong  nitric  acid.  Evaporate  the  acid.  Cover 
the  crucible  and  carefully  heat  it  until  all  tendency  on  the  part 
of  the  oxide  to  decrepitate  has  passed.  Remove  the  cover  and 
heat  the  material  exposed  to  the  air  for  an  hour  over  a  Bunsen 
burner.  Weigh,  and  again  heat  for  half  an  hour  in  order  to  ascer- 
tain whether  the  weight  is  constant.  Compare  the  weight  of  the 
iron  in  the  oxide  with  that  found  by  the  volumetric  method. 

It  is  preferred  by  many  to  use  iron  wire  in  which  the  iron  has 
been  determined  gravimetrically  for  the  standardization  of  per- 
manganate solutions.  It  is  to  be  observed  in  regard  to  this 
practice  that  the  carbon  in  the  iron  may  give  rise  to  errors. 

Owing  to  the  volatility  of  ferric  chloride,  ferric  hydroxide 
which  is  precipitated  from  solutions  containing  chlorides  must 
be  washed  until  the  filtrate  is  free  from  chlorine. 


464  QUANTITATIVE  EXERCISES 

Iron  in  the  ferrous  condition  may  also  be  determined  volu- 
metrically  by  means  of  a  standard  solution  of  potassium  dichro- 
mate.  The  solution  of  the  ferrous  salt  is  acidified  with  sulphuric 
acid  and  treated  with  the  dichromate  until  a  drop  of  the  liquid 
taken  out  upon  a  glass  rod  and  applied  to  a  drop  of  a  solution 
of  potassium  ferri cyanide  upon  a  white  porcelain  surface  no 
longer  gives  a  blue  color. 

The  method  gives  good  results  in  the  presence  of  chlorides. 
The  ferricyanide  used  to  detect  the  completion  of  the  reaction 
must,  of  course,  be  free  from  potassium  ferrocyanide. 


EXERCISE  LII 

DETERMINATION  OF  FERROUS  AND  FERRIC  IRON 
IN  A  SILICATE 

Prepare  2  or  3  grams  of  manganese  dioxide  by  slowly  stir- 
ring a  dilute  solution  of  manganese  sulphate  into  a  hot  solu- 
tion of  potassium  permanganate  which  contains  more  of  the 
permanganate  than  is  required  for  the  oxidation  of  the  manga- 
nese. Filter,  wash  the  precipitate,  and  mix  it  with  aqueous 
hydrofluoric  acid  in  a  platinum  retort.  After  the  acid  has  been 
in  contact  with  the  oxide  for  a  few  hours,  slowly  distill  it  into 
a  platinum  receiver  which  contains  a  little  water  and  is  kept 
cool  by  ice  water.  Dilute  a  little  of  the  distillate  in  a  platinum 
dish  and  ascertain  whether  it  reduces  permanganate.  If  it  does, 
return  the  remainder  of  the  acid  to  the  retort  and  allow  it  to 
stand  for  a  longer  time  in  contact  with  the  oxide. 

The  apparatus  in  which  the  silicate  is  to  be  decomposed  con- 
sists of  two  glass  funnels  of  equal  size,  the  rims  of  which  are 
held  together  by  a  broad  rubber  band.  One  of  them  is  placed 
in  a  hot-water  funnel  and  connected  at  its  lower  end  with  a 
carbon  dioxide  generator.  The  stem  of  the  other  is  cut  off  at 
a  point  about  1 0  or  1 5  mm.  from  the  conical  part. 


POTASSIUM  PERMANGANATE  465 

Place  about  half  a  gram  of  finely  pulverized  biotite  in  a  small 
platinum  dish  and  add  to  it  5  or  6  cc.  of  dilute  sulphuric  acid. 
Put  the  dish  on  a  small  triangle  in  the  bottom  of  the  lower  fun- 
nel and  conduct  carbon  dioxide  through  the  apparatus  until  it  is 
believed  that  the  air  has  been  displaced.  Heat  the  water  funnel 
and  pour  10  or  15  cc.  of  the  purified  hydrofluoric  acid  down  a  stout 
platinum  wire  through  the  orifice  in  the  upper  funnel.  Diminish 
the  flow  of  carbon  dioxide  as  much  as  is  consistent  with  the  pro- 
tection of  the  contents  of  the  dish  from  the  air.  Occasionally  stir 
the  decomposing  silicate  with  the  platinum  wire  and  continue  to 
heat  until  the  decomposition  appears  to  be  complete.  Transfer 
the  dish  to  a  beaker  containing  boiling  water  and  add  a  little  acid 
sodium  carbonate.  Remove  the  dish  and  rinse  both  it  and  the 
platinum  wire  into  the  beaker  with  hot  water.  Add  a  little  more 
sodium  carbonate  and  titrate  with  permanganate.  Do  not  use 
the  platinum  wire  as  a  stirring  rod  during  the  titration. 

Wind  a  platinum  wire  spirally  about  a  rod  of  zinc  and  insert 
the  pair  in  the  solution,  leaving  one  end  of  the  wire  above  the 
surface.  Heat  gently  and  stir  until  a  drop  of  the  liquid  fails 
to  impart  a  red  color  to  a  solution  of  ammonium  sulphocyanate. 
Allow  the  solution  to  cool,  adding  from  time  to  time  small 
portions  of  sodium  carbonate.  When  it  is  cold  remove  the  zinc 
and  rinse  it  back  into  the  beaker.  Titrate  with  permanganate. 
Repeat  the  reduction  and  titration  in  the  same  manner  as  before, 
adding  more  sulphuric  acid  if  necessary. 

The  first  operation  gives  the  ferrous,  and  the  second  the 
total,  iron  in  the  silicate.  The  difference  between  the  two  is, 
of  course,  the  iron  which  is  in  the  ferric  condition. 

The  zinc  which  is  used  in  the  reduction  must  be  free  from 
arsenic  and  antimony  and  also  from  metals  whose  salts  reduce 
permanganate.  It  must  likewise  be  free  from  lead,  since  that 
metal  forms  with  zinc  an  alloy  which  dissolves  very  slowly  and 
often  becomes  detached  from  the  rod. 

The  solution  must  not  be  allowed  to  become  neutral,  since  in 
that  condition  the  iron  is  deposited  in  metallic  form. 


466  QUANTITATIVE  EXERCISES 


EXERCISE  LIII 

DETERMINATION  AND  SEPARATION  OF  IRON  AND 
ALUMINIUM 

1.  Weigh  out  about  5  grams  of  ferric  ammonium  sulphate 
(NH4Fe  (SO4)2  12  H2O),  and  about  10  grams  of  pure  aluminium 
ammonium  sulphate  (NH4A1  (SO4)2  12  H2O).    Dissolve  the  salts 
in  water.     Transfer  the  solution  to  a  quarter-liter  flask  and  fill 
to  the  mark  with  water.     Measure  50  cc.  of  the  solution  into  a 
beaker  glass.     Add  water  and  dilute  sulphuric  acid.     Reduce 
the  ferric  salt  with  zinc  and  platinum,  as  directed  in  the  pre- 
ceding   exercise,  and    determine   the    iron    with  the  standard 
solution  of  permanganate. 

2.  Measure  another  50-cc.   portion  of   the  solution    into  a 
porcelain  dish.     Add  water  and  about  10  cc.  of  dilute  hydro- 
chloric acid.     Heat  nearly  to  the  boiling  point  and  then  slowly 
add  ammonia,  stirring  constantly  until  the  liquid  is  very  faintly 
ammoniacal.     Continue  to  heat  for  half  an  hour  and  then  deter- 
mine whether  the  liquid  is  still  slightly  alkaline.     If  it  is  not, 
make  it  so  with  the  least  possible  quantity  of  ammonia.    Decant 
the  clear  portion  of  the  liquid  through  a  paper  filter.     Add 
boiling  water  to  the  residue  in  the  beaker.     Allow  the  precipi- 
tate to  subside  and  then  decant  again.     Bring  the  precipitate 
upon  the  paper  and  wash  two  or  three  times  with  hot  water. 
Dissolve  the  mixture  of  hydroxides  in  the  filter  with  dilute 
hydrochloric  acid.     Wash  the  paper  thoroughly,  collecting  the 
filtrate  in  the  porcelain  dish  which  was  used  for  the  precipita- 
tion.    Heat  the  solution  and  precipitate  as  before  with  the 
slightest  possible  excess  of  ammonia.     Collect  the  precipitate 
upon  a  filter  and  wash  with  hot  water  until  the  filtrate  gives 
no  reaction  for  chlorine.     Proceed  with  the  filter  and  its  con- 
tents as  directed  under  Exercise  LI,  taking  all  possible  pre- 
cautions to  avoid  any  spattering  of  the  oxides  in  the  crucible. 
From  the  weight  of  the  mixed  oxides  and  the  known  weight  of 


POTASSIUM  PERMAKGAHATE  467 

the  iron  oxide,  calculate  the  weight  of  the  aluminium  oxide. 
Fuse  some  acid  potassium  sulphate  in  a  platinum  crucible  or 
dish,  applying  just  enough  heat  to  keep  the  salt  in  the  molten 
condition.  When  the  fused  mass  becomes  perfectly  tranquil, 
pour  it  into  a  cold  platinum  dish  or  upon  a  piece  of  platinum 
foil.  Pulverize  the  potassium  pyrosulphate  and  add  2  or  3 
grams  of  the  powder  to  the  crucible  containing  the,  mixture 
of  iron  and  aluminium  oxides.  Heat  the  crucible  over  a 
Bunsen  burner  until  the  oxides  are  completely  dissolved  in  the 
fused  pyrosulphate,  keeping  the  temperature  just  high  enough 
to  produce  a  constant  but  very  slow  evolution  of  sulphur  tri- 
oxide.  Place  the  crucible  when  cold  in  a  beaker.  Add  cold 
water,  also  a  little  dilute  sulphuric  acid,  and  stir  until  the 
fused  sulphates  are  dissolved.  Reduce  with  zinc  and  platinum 
and  determine  the  iron  by  permanganate. 

The  iron  in  this  mixture  of  oxides  may  also  be  determined 
by  reducing  in  a  current  of  dry  hydrogen,  dissolving  the  metallic 
iron  in  dilute  sulphuric  acid,  and  titrating  with  permanganate. 

3.  Place  about  10  cc.  of  water  in  a  large  silver  dish  and  add 
thin  shavings  of  metallic  sodium,  one  by  one,  until  about  3  grams 
of  the  metal  have  been  dissolved.  Evaporate  the  excess  of  water 
quite  rapidly.  Fuse  the  residue  and  continue  to  heat  until  the 
molten  hydroxide  becomes  tranquil.  Place  the  dish  under  a 
bell  jar  until  needed. 

Measure  50  cc.  of  the  solution  of  sulphates  into  a  porcelain 
dish.  Precipitate  the  iron  and  aluminium  with  a  very  slight 
excess  of  ammonia,  as  previously  directed.  Collect  the  precipi- 
tate upon  a  filter  and  wash  it  with  hot  water.  Dry  the  filter 
at  100°.  Open  it  over  a  piece  of  glazed  paper  and  scrape  off 
the  precipitate.  Burn  the  paper  in  a  platinum  wire  and  let  the 
ash  fall  into  a  porcelain  or  platinum  dish.  Place  the  precipi- 
tate and  ash  on  the  center  of  the  sodium  hydroxide  in  the  silver 
dish.  Heat  the  dish  cautiously  until  all  tendency  on  the  part 
of  the  contents  to  spatter  ceases.  Keep  the  hydroxide  in  the 
molten  condition  for  about  half  an  hour.  Allow  the  dish  and 


468  QUANTITATIVE   EXERCISES 

its  contents  to  cool.  Add  quickly  and  at  one  time  about  300  cc. 
of  water.  Keep  .the  solution  hot  for  an  hour  in  order  to  effect 
a  complete  separation  of  the  iron.  Decant  through  a  filter 
and  wash  the  precipitate  several  times  by  decantation,  using  hot 
water.  Bring  it  upon  the  filter  and  wash  with  boiling  water 
until  the  filtrate  shows  a  neutral  reaction.  Dissolve  the  pre- 
cipitate in  hydrochloric  acid.  Reprecipitate  with  ammonia  from 
the  hot  solution,  and  proceed  as  previously  directed  to  obtain 
the  weight  of  the  ferric  oxide. 

Acidify  the  filtrate  which  contains  the  aluminium  with  nitric 
or  hydrochloric  acid.  Precipitate  from  the  hot  solution  with 
the  least  possible  excess  of  ammonia.  Filter,  and  wash  with 
hot  water.  Dissolve  the  precipitate  with  hydrochloric  acid. 
Precipitate  again  with  ammonia.  Filter,  and  wash  with  hot 
water  until  the  filtrate  gives  no  reaction  for  chlorine.  Roll  up 
the  paper  and  precipitate  and  place  the  roll  in  a  platinum  cru- 
cible. Heat  cautiously  over  a  Bunsen  burner  until  the  paper 
is  burned  and  the  residue  shows  no  inclination  to  decrepitate, 
and  finally  to  constant  weight  over  the  blast  lamp. 

The  reprecipitation  of  the  iron  and  aluminium  by  ammonia 
is  necessary  in  order  to  free  the  hydroxides  from  alkali. 


EXERCISE  LIV 

DETERMINATION  OF  MANGANESE  BY  POTASSIUM 

1T.KMANGANATE 

BY  VOLHARD'S  MKTHOD 

Dissolve  about  10  grams  of  manganese  sulphate  in  a  quarter 
liter  of  water  and  test  the  solution  for  iron.  If  no  iron  is 
found,  dilute  to  half  a  liter.  If  iron  is  present,  add  2  or  3  cc. 
of  dilute  nitric  acid  and  boil.  Take  out  about  one-tenth  of  the 
solution  and  precipitate  the  manganese  in  it  with  ammonium 
carbonate  to  which  ammonia  has  been  added.  Filter,  wash  the 
precipitate,  and  add  it  to  the  main  solution  of  the  sulphate. 


roTASSlfM    IT.RMAMiANATK  4(\\) 

Heat  tin1  solution  to  the  boiling  point,  stirring  it  constantly. 
Filter,  ami  dilute  the  filtrate  to  half  a  liter. 

DissoUe  in  a  quarter  litor  of  water  about  -f>  grains  of  /.ine 
sulphate  which  does  not  reduce  permanganate  in  solutions 
acidified  with  sulphuric  acid.  If  a  suitable  specimen  of  the 
sulphate  is  not  obtainable,  heat  to  redness  in  a  platinum  dish, 
in  a  inutile,  somewhat  more  than  the  requisite  amount  of  com- 
mercial /inc  oxide,  and  add  it  to  a  quantity  of  hot  dilute  sul- 
phuric acid  which  is  insufficient  to  diss61ve  the  whole  of  the 
oxide.  Filter,  and  dilute  the  filtrate  so  that  each  cubic  centi- 
meter of  the  solution  shall  contain  about  one-tenth  of  a  gram 
of  zinc  sulphate. 

Measure  10  cc.  of  the  manganese  sulphate  solution  into  a 
three-quarter  liter  flask.  Add  10  cc.  of  the  zinc  sulphate  solu- 
tion and  2  or  3  drops  of  dilute  nitric  acid.  Dilute  with 
water,  which  does  not  reduce  permanganate,  to  about  250  or 
800  cc.  Heat  to  the  boiling  point  and  then  add  rapidly  nearly 
as  much  standard  potassium  permanganate  as  it  is  thought 
will  be  required  to  precipitate  the  manganese  according  to  the 

equation : 

Mn2O7  +  3  MnO  =  5  MnO2. 

Shake  the  flask  vigorously  and  then  add  more  cautiously  the 
remainder  of  the  permanganate  required  to  produce  a  perma- 
nent color,  shaking  the  flask  from  time  to  time.  If  the  solu- 
tion is  turbid  and  slow  to  clear  in  consequence  of  the  fineness 
of  the  precipitate,  it  may  be  heated  again,  but  not  to  the  boil- 
ing point.  Having  found  by  the  first  experiment  very  nearly 
the  amount  of  permanganate  required,  repeat  the  determination 
with  another  equal  portion  of  the  sulphate,  adding  at  once,  to 
within  one-  or  two-tenths  of  a  cubic  centimeter,  all  of  the  per- 
manganate which  the  previous  experiment  has  shown  to  be 
necessary,  and  finishing  the  determination  with  greater  caution 
than  before. 

Test  the  constancy  of  the  results  by  several  repetitions  of  the 
experiment,  using  varying  quantities  of  the  manganese  sulphate. 


170  QUANTITATIVE  KM-HICISES 

Proceed  also  in  tho  following  manner:  Measure  out,  a  quan- 
tity of  tlio  sulphate  solution  and  prepare  it  for  the  precipitation 
of  (lie  manganese  as  previously  directed.  Measure  into  another 
Ilask  of  sullie.iont  si/e  (lie  quantity  of  permanganate  which  the 
previous  work  has  shown  to  be  required  by  the  given  volume  of 
sulphate,  and  warm  it  upon  the  water  bath.  When  the  solu- 
tion of  the  sulphate  is  boiling  hot  and  that  of  the  permanganate 
is  warm,  pour  the  former  into  the  latter,  rinsing  the  emptied 
Ilask  with  boiling  water.  During  the  transfer,  the  mixing  of 
the  liquids  should  he  facilitated  as  much  as  possible  by  shaking 
and  rotating  the  Ilask,  and  whenever  the  pouring  is  interrupted 
for  this  purpose,  the  flask  containing  the  remainder  of  the  sul- 
phate should  l)e  again  placed  over  the  flame.  It  will  be  found 
that,  a  further  addition  of  permanganate  is  required  in  order  to 
produce  a  permanent  color. 

III'.mV.TION  OF  PERMANGANIC  ACID  BY  THE  OXIDES 
OF    M  AN<;  AN  USE 

Measure  50  cc.  of  tho  standard  permanganate  solution  into  a 
Ilask  and  add  to  it  tho  quantity  of  manganous  sulphate  which 
is  required  to  reduce  one-half  of  the  permanganate  to  MnO.,. 
Immerse  the  body  of  the  ilask  in  boiling  water  and  heat  until 
the  color  of  the  permanganate  disappears,  which  may  require 
from  "2  to  3  hours.  Now  determine  the  remaining  "active" 
o\  \  gen  by  adding  a  measured  but  excessive  quantity  of  a  stand- 
ard solution  of  oxalic  acid  or  of  tetroxalate,  also  a  little  sul- 
phuric acid  and  titrating  to  color — when  the  manganese  oxides 
have  disappeared  —  with  potassium  permanganate. 

It  will  be  found  that,  about  throe-tenths  of  the  oxygon  has  dis- 
appeared, in  other  words,  that  the  excess  of  the  permanganate 
has  been  reduced  with  the  evolution  of  free  oxygen  by  the  oxide 
precipitated  by  the  action  of  a  portion  of  the  permanganate  on 
the  manganous  sulphate  : 

2  IIMn04  =  1 1/)  +  2  Mn02  +  3  O. 


POTASSIUM  PERMANGANATE  171 

If  (ho  determination  is  made  immediately  after  the  disappear- 
ance of  the  color  of  the  permanganate,  the  quantity  of  oxygen 
found  will  be  very  nearly  that  which  would  be  obtained  if  all 
the  manganese  in  the  solution  were  in  the  dioxide  condition. 
If  the  determination  is  postponed  for  any  length  of  time,  the 
quantity  of  oxygen  found  will  bo  somewhat  less. 


EXERCISE  LV 

DKTKKMINATION  OF  MANC ANKSK   AS  PYROPIH  >SI»II  AIT, 
BY  GIBBS'  METHOD 

Acidify  20-cc.  portions  of  the  manganese  sulphate  solution 
used  in  the  previous  exercise  with  8  or  4  cc.  of  dilute  hydro- 
chloric acid.  Heat  to  the  boiling  point,  and  add  a  moderate 
excess  of  ammonium  phosphate  (H(NH4),2PO4)  in  solution. 
Add  ammonia,  drop  by  drop,  with  constant  stirring  until  a  per- 
manent precipitate  forms.  Stop  adding  the  ammonia,  but  con- 
tinue the  stirring  at  the  boiling  temperature  until  the  precipitate 
assumes  a  silky  crystalline  appearance.  Add  more  ammonia, 
stirring  vigorously  after  the  addition  of  each  drop,  until  all  the 
manganese  has  been  precipitated  and  the  whole  of  the  precipi- 
tate has  become  crystalline  in  appearance.  Add  a  few  more 
drops  of  ammonia.  Place  the  beaker  in  ice  water  and  let  it 
remain  there  until  the  solution  is  perfectly  cold.  Filter  through 
paper  and  wash  the  precipitate  with  a  cold,  faintly  ammoniaeal, 
10  per  cent  solution  of  ammonium  nitrate.  The  subsequent 
treatment  of  the  precipitate  is  the  same  as  in  the  determination 
of  magnesium  or  phosphoric  acid  as  magnesium  pyrophosphale. 
Test  the  filtrate  for  manganese.  The  precipitate  has  the  com- 
position NII4MnPO4,  and  like  the  corresponding  magnesium 
salt,  it  is  eonyerted  by  heat  into  the  pyrophosphate  MiiyP./ >7. 


472  QUANTITATIVE  EXERCISES 

EXERCISE  LVI 
DETERMINATION  OF  HYDROGEN  PEROXIDE 

If  the  solution  of  peroxide  is  concentrated,  dilute  10  cc.  of 
it  to  100  cc.  Measure  off  10-cc.  .portions  of  the  diluted  solu- 
tion, add  water  and  dilute  sulphuric  acid,  and  titrate  with  per- 
manganate. There  are  in  use  two  ways  of  expressing  the  value 
of  hydrogen  peroxide  solutions.  One  gives  the  percentage  by 
weight,  and  the  other  the  volume  of  free  oxygen  which  a  cubic 
centimeter  of  the  solution  would  yield  if  the  peroxide  were 
decomposed  into  water  and  oxygen. 

Another  method  (Lunge's)  for  the  determination  of  hydrogen 
peroxide  is  based  on  the  following  reaction : 

CaCl20  +  H202  =  CaCl2  +  H2O  +  O2. 

The  free  oxygen  is  determined  by  direct  measurement  or  by 
absorption. 

EXERCISE  LVII 
DETERMINATION  OF  NITROUS  ACID 

Weigh  off  about  5  grams  of  the  commercial  potassium  nitrite, 
taking  care  to  secure  an  average  sample  of  the  whole  lot  from 
which  the  material  is  taken.  Dissolve  in  water  and  dilute  to 
a  liter.  Measure  10-cc.  portions  into  beakers.  Add  to  each  a 
measured  but  excessive  quantity  of  permanganate,  remember- 
ing that  in  the  alkaline  solutions  only  1^  atoms  of  oxygen 
per  molecule  of  permanganate  will  be  available  for  the  oxi- 
dation of  the  nitrite.  Warm  for  half  an  hour  upon  the  water 
bath.  In  the  meantime  prepare  a  solution  of  oxalic  acid  whose 
strength  is  nearly  equal  to  that  of  the  permanganate.  Add 
to  the  contents  of  each  beaker  the  volume  of  oxalic  acid  which 
is  equivalent  to  the  volume  of  permanganate  which  was  added 
to  the  nitrite.  Acidify  with  sulphuric  acid.  When  the  solu- 
tion lias  become  clear  and  colorless,  titrate  to  color  with  the 


POTASSir.M   PERMANGANATE  473 

permanganate.  The  quantity  of  permanganate  used  in  the  last 
case  is  equivalent  to  the  nitrite. 

Measure  off  10  cc.  of  the  nitrite  solution.  Dilute  with  300  cc. 
of  water.  Make  the  solution  very  slightly  acid  with  sulphuric 
acid.  Add  permanganate  until  the  nitrous  acid  is  nearly  all 
oxidized,  then  make  the  solution  more  strongly  acid,  and  com- 
plete the  titration.  Owing  to  the  limited  solubility  of  nitric 
oxide  in  water,  the  solution  of  the  nitrite  must  be  very  dilute, 
containing  not  more  than  1  part  of  nitrous  acid  anhydride  to 
5000  parts  of  water.  The  end  reaction  is  somewhat  indefinite  in 
consequence  of  the  slowness  with  which  nitric  oxide  is  oxidized 
by  permanganic  acid. 

Nitrogen  dioxide  may  also  be  determined  by  permanganate. 
For  this  purpose  a  measured  portion  of  the  solution  containing 
it,  e.g.  fuming  nitric  acid,  is  slowly  stirred  into  a  large  quan- 
tity of  cold  water  and  the  dilute  solution  titrated.  Each 
molecule  of  the  nitrous  anhydride  found  is  equivalent  to  four 
molecules  of  NO2,  since  2  NO2  +  H2O  =  HNO3  +  HNOa. 


CHAPTER  XXI 

THE  ELECTROLYTIC  DETERMINATION  OF  METALS 
ELECTRICAL  UNITS  AND  THEIE  KELATIONS 

1.  THE  AMPERE  AND  THE  COULOMB 

The  ampere  is  the  unit  of  the  quantity  of  electricity  flowing 
through  a  circuit.  It  is  denned  as  the  strength  of  the  current 
which  a  unit  of  electromotive  force  (one  volt)  will  cause  to  flow 
through  a  conductor  whose  resistance  is  unity  (one  ohm).  It  is 
also  the  quantity  of  current  which,  under  certain  conditions, 
will  precipitate  per  second  0.3292  milligram  of  copper  or  1.1172 
milligrams  of  silver. 

The  coulomb  is  the  quantity  of  electricity  which  flows  through 
a  circuit  during  one  second  when  the  strength  of  the  current  is 
one  ampere. 

The  ampere-hour  is  the  quantity  of  electricity  which  flows 
through  a  circuit  during  one  hour  when  the  strength  of  the 
current  is  one  ampere.  It  is  therefore  equal  to  3600  coulombs. 

The  current  strength  is  the  same  in  all  parts  of  an  undivided 
circuit. 

2.  THE  OHM 

The  ohm  is  the  unit  of  the  resistance  to  the  flow  of  a  current 
in  a  conductor.  It  is  equal  to  the  resistance  offered  at  0°  by  a 
column  of  mercury  whose  length  is  106.3  centimeters  and  whose 
cross  section  is  equal  to  one  square  millimeter.  The  resistance 
of  a  conductor  of  any  given  material  is  directly  proportional  to 
its  length  and  inversely  proportional  to  the  area  of  its  cross 
section. 

474 


ELECTROLYTIC  DETERMINATION  OF  METALS        475 

The  specific  resistance  of  a  substance  is  the  resistance  found 
in  a  conductor  made  from  that  material  whose  length  is  one 
centimeter  and  whose  cross  section  has  an  area  of  one  square 
centimeter. 


The  resistance  of  any  conductor  is  equal  to  the  specific  resist- 
ance of  the  material  of  which  it  is  made  multiplied  by  its  length 
and  divided  by  the  area  of  its  cross  section : 

_  specific  resistance  X  length 
area  of  cross  section 

With  rising  temperature  the  specific  resistance  of  metals  in- 
creases, while  that  of  carbon,  liquids,  and  generally  of  solutions, 
diminishes.  The  specific  resistance  of  solutions  also  varies  with 
concentration. 

The  conductivity  of  a  material  is  the  reciprocal  of  its  resist- 
ance. 

If  a  current  is  given  two  or  more  paths  between  two  points 
in  a  circuit,  it  will  distribute  itself  among  them  in  quantities 
which  are  inversely  proportional  to  the  resistance  or  directly 
proportional  to  the  conductivity  of  the  several  conductors,  and 
the  total  conductivity  of  the  divided  line  will  be  the  sum  of 
the  conductivities  of  the  individual  lines.  If,  for  example,  the 
resistances  of  three  parallel  lines  are  10,  20,  and  30  ohms  respec- 
tively, their  separate  conductivities  will  be  -j1^,  ^,  and  ^,  and 
the  total  conductivity  will  be  J^,  or  0.18333;  while  the  resist- 
ance of  the  lines,  taken  together,  will  be  ,  or  5.45  ohms. 

O.loooo 


3.  THE  VOLT 

The  volt  is  the  unit  of  electromotive  force.  It  is  the  electrical 
pressure  which  will  send  one  ampere  of  current  through  a  con- 
ductor whose  resistance  is  one  ohm. 


476  QUANTITATIVE  EXERCISES 


4.  OHM'S  LAW 

Ohm's  law  of   the  current  is   expressed   by  the  following 
equation : 

r    E- 

=  R> 
in  which 

C  represents  the  current  in  amperes, 
E,  the  electromotive  force  in  volts,  and 
R,  the  resistance  of  the  circuit  in  ohms. 
It  will  be  seen  that  if  any  two  of  these  values  are  known,  the 
third  can  be  calculated,  for 

2F=  CR  and  R  =  -. 


Determination  of  the  Strength  of  the  Current 

The  strength  of  the  current  is  ascertained  by  measuring  either 
its  chemical  or  its  electro-magnetic  effects. 

Instruments  for  the  former  purpose  are  known  as  voltameters, 
and  these  are  classified  as  gas  (oxy-hydrogen  or  hydrogen),  or 
as  weight  voltameters,  according  as  the  gas  resulting  from  an 
electrolysis  of  definite  duration  is  measured,  or  the  metal  depos- 
ited on  an  electrode  is  weighed.  The  quantity  of  gas  which  is 
liberated  per  second  in  the  oxy-hydrogen  voltameter  and  the 
weight  of  metal  which  is  precipitated  in  the  silver  or  copper 
voltameter  in  the  same  unit  of  time  have,  for  fixed  conditions, 
perfectly  definite  equivalents  in  coulombs  of  current;  hence,  by 
noting  the  time  of  an  electrolysis  and  measuring  or  weighing 
the  products,  the  strength  of  the  current  can  be  ascertained. 
But  voltameters  are  little  used  in  the  laboratory,  owing  princi- 
pally to  the  long  time  required  for  a  determination. 

Instruments  depending  on  the  electro-magnetic  effects  of  the 
current  are  likewise  of  two  kinds.  In  one  of  these  a  magnet  is 
displaced  by  the  current  flowing  in  its  vicinity,  while  in  the 
other  a  bar  or  needle  of  soft  iron  is  employed  which  becomes  a 


ELECTROLYTIC   DETERMINATION  OF  METALS        477 

magnet  under  the  influence  of  the  current  and  is  then  displaced. 
Instruments  of  the  first  kind  are  known  as  galvanometers,  and 
of  the  second  as  amperemeters  or  ammeters.  Only  amperemeters 
are  used  in  ordinary  electrolytic  work.  These  are  provided 
with  a  scale  for  the  direct  reading  of  the  amperage  and  often 
with  two  scales,  one  for  small  and  the  other  for  large  currents. 
Amperemeters  have  a  small  resistance  and  are  inserted  in  the 
circuit. 

Determination  of  Voltage 

The  measurement  of  the  fall  or  difference  of  potential  between 
any  two  points  in  a  circuit  is  usually  made  for  practical  purposes 
by  means  of  a  so-called  voltmeter.  This  instrument  resembles 
the  amperemeter,  but  differs  from  the  latter  in  that  its  resistance 
is  so  high  that  when  it  is  inserted  in  a  shunt  between  the  two 
points,  only  a  minute  fraction  of  the  current  passes  through  the 
voltmeter  ;  its  employment  does  not,  therefore,  materially  affect 
the  conductivity  of  the  line  as  a  whole.  An  instrument  meas- 
uring from  0  to  150  volts  has  a  resistance  of  about  12,500  ohms. 
The  range  of  the  voltmeter  is  greatly  enlarged  by  using  "  mul- 
tipliers," i.e.  standard  resistances  which  are  placed  in  series  with 
it  in  the  shunt  circuit  and  thus  increase  the  value  of  the  scale 
divisions  by  definite  multiples.  .  . 

Having  ascertained  the  quantity  of  the  current  flowing 
through  the  circuit,  and  the  fall  of  potential  between  any  two 
points  on  the  same,  the  resistance  in  the  line  between  them  is 
found  by  the  equation 


5.  THE  JOULE 

The  joule  is  the  electrical  unit  of  energy.  It  is  equivalent  to 
the  quantity  of  heat  developed  in  a  circuit  by  one  coulomb  of  a 
current  whose  electromotive  force  is  one  volt  : 

1  joule  =  1  coulomb  x  1  volt. 


478  QUANTITATIVE  EXERCISES 

According  to  Joule's  law,  the  heat  in  calories  which  is  devel- 
oped in  a  circuit  with  a  resistance  It,  by  a  current  C\  in  the 
time  £,  is  expressed  by  the  equation: 

calories  =  C*Rt  x  0.2381  =  CR  x  Ct  x  0.2381. 

But  according  to  Ohm's  law  CR  =  E,  the  electromotive  force 
of  the  current ;  hence 

calories  =  E  x  Ct  x  0.2381. 

If  C  is  one  ampere  and  t  is  one   second,  Ct  is  the  coulomb; 
therefore 

calories  =  E  x  coulombs  (joules)  x  0.2381. 

calories 

05881- 

1  joule     =  0.2381  calorie. 
1  calorie  =  4.2  joules. 


6.  THE  WATT 

The  watt  is  the  electrical  unit  of  work.  It  is  the  mechanical 
equivalent  of  the  joule. 

Since  the  mechanical  equivalent  of  the  calorie  is  3.0968  foot 
pounds  per  second,  that  of  the  joule  ( =  0.2381  calorie)  is 
0.73732  foot  pound.  The  watt,  therefore,  is  0.73732  foot  pound 
per  second,  and  745.94  watts  equal  550  foot  pounds  per  second, 
or  one  horse  power. 

The  work,  in  watts,  which  can  be  accomplished  by  any  cur- 
rent is  the  product  of  its  quantity  in  amperes  by  its  pressure  in 

volts.     Expressed  in  horse  power,  it  is  ^77^77  '     ^n  ^e  case  °^ 

powerful  currents,  e.g.  those  of  large  dynamos,  it  is  customary 
to  employ  the  kilowatt  (=  1000  watts  =  1.3406  horse  power)  as 
the  unit  of  work. 


ELECTROLYTIC  DETERMINATION  OF  METALS        479 

SOURCES  OF  CURRENT 

The  general  employment  of  electricity  for  lighting,  mechani- 
cal, and  other  purposes  has  brought  the  current  in  great  abun- 
dance within  the  reach  of  nearly  all  laboratories.  The  current 
as  it  is  obtained  from  the  town  supplies  has,  however,  a  far  too 
high  electromotive  force  for  the  ordinary  electrolytic  processes, 
and  it  must  be  transformed  to  a  lower  voltage.  This  is  accom- 
plished by  means  of  dynamos,  or  by  means  of  storage  batteries 
(accumulators)  which  are  charged  either  directly  from  the  street 
current  or  by  the  current  of  a  small  dynamo  located  in  the 
building  and  driven  by  the  street  current. 

The  highest  electromotive  force  required  for  the  quantitative 
precipitation  and  separation  of  the  metals  and  for  most  other 
electrolytic  work  does  not  exceed  10  volts,  while  the  pressures  of 
the  currents  received  from  the  street  are  ordinarily  110,  115, 
220,  230,  and  even  500  volts.  The  economical  transformation 
of  these  to  currents  of  the  low  voltages  which  are  adapted  to 
laboratory  purposes  is  a  matter  of  much  importance. 

The  too  common  practice  of  charging  small  storage  batteries 
by  inserting  resistance  in  the  line  from  the  street  until  the  cur- 
rent normal  for  the  particular  type  of  cell  is  obtained  is  attended 
by  an  enormous  loss  of  energy. 

The  voltage  of  the  charging  current  should  only  slightly 
exceed  the  counter  electromotive  force  of  the  battery.  All  ex- 
cess of  the  former  over  the  latter  is  wasted.  Suppose,  by  way  of 
illustration,  that  a  battery  consisting  of  four  cells  with  a  nor- 
mal charging  rate  of  12.5  amperes  is  to  be  charged.  Arranged 
in  series,  the  batteiy  toward  the  close  of  the  charging  period  will 
have  an  electromotive  force  of  10  volts,  2.5  volts  per  cell.  The 
maximum  work  to  be  done  is,  therefore,  10  x  12.5  =  125  watts, 
or  0.168  horse  power.  Suppose,  however,  that  the  battery  is 
charged  from  a  line  on  which  the  pressure  is  115  volts,  resist- 
ance being  inserted  until  a  current  of  12.5  amperes  is  obtained, 
the  work  accomplished  in  charging  the  battery  or  lost  in  heating 


480 


QUANTITATIVE  EXERCISES 


the  resistance  will  be  115  x  12.5  =  1437.5  watts,  or  1.927  horse 
power.  In  other  words,  the  waste  of  energy  amounts  to  more 
than  91  per  cent.  As  the  number  of  cells  in  series  increases, 
the  waste  diminishes.  But  to  charge  economically  with  a  cur- 
rent of  115  volts,  the  number  of  cells  which  must  be  placed  in 
series  is,  approximately,  ^f-  =  46.  Similarly,  if  the  pressure 
upon  the  charging  line  is  230  volts,  the  number  of  cells  should 
be  nearly  92,  etc.  Such  large  batteries  are,  however,  not  ordi- 
narily necessary,  and  even  when  currents  equal  to  their  capaci- 
ties are  needed,  it  is  more  economical,  and  less  troublesome,  to 
employ  suitable  dynamos  unless  great  steadiness  of  current  pres- 
sure is  required. 

If  a  dynamo  is  to  be  used  and  there  is  no  opportunity  to 
utilize  power  employed  for  other  purposes  in  the  building,  a 


FIG.  67 

motor  to  run  it  must  be  provided.  A  motordynamo,  or  rotary 
transformer  like  the  Crocker- Wheeler  machine  shown  in  Fig.  67, 
is  in  every  way  satisfactory.  These  machines  are  built  in  a  great 
variety  of  sizes  from  one-sixth  horse  power  upwards.  In  some 
of  them  the  motor  and  dynamo  armatures  are  separately  wound 
upon  a  single  shaft,  each  having  its  own  field.  Either  end  may 
operate  as  a  motor  or  as  a  dynamo,  or  if,  as  in  some  cases  where 
power  is  available,  the  shaft  is  extended  and  provided  with  a 
pulley,  the  machines  may  be  employed  as  double  dynamos  to 
furnish  currents  of  different  pressures.  The  armature  of  the 


ELECTROLYTIC  DETERMINATION  OF  METALS        481 

motor  is  wound  to  suit  the  voltage  of  the  street  current,  while 
that  of  the  dynamo  is  wound  to  meet  the  needs  of  the  laboratory. 
The  dynamo  is  provided  with  a  field  rheostat  by  which  the  volt- 
age of  its  current  may  be  economically  cut  down  step  by  step  to 
any  required  extent.  The  size  of  the  machine  will,  of  course, 
depend  upon  the  amount  of  work  to  be  done,  while  the  charac- 
ter of  the  dynamo  winding  will  be  determined  by  the  nature 
of  that  work.  If  the  motor  is  one  horse  power,  the  output 
of  the  dynamo  will  probably  be  somewhat  over  600  watts,  and 
the  latter  may  be  wound  so  as  to  secure  any  desired  relation  of 
pressure  to  current,  provided,  of  course,  the  product  of  the  two 
does  not  exceed  the  maximum  rated  output.  For  example,  if 
the  output  of  a  dynamo  is  600  watts,  it  may  be  wound  so  as  to 
give  a  current  of  60  volts  and  10  amperes,  10  volts  and  60 
amperes,  6  volts  and  100  amperes,  20  volts  and  30  amperes,  or 
30  volts  and  20  amperes,  etc.  The  maximum  current  (but  not 
more)  may  be  generated  at  any  of  the  lower  voltages  which  are 
secured  by  throwing  resistance  into  the  field. 

A  storage  battery  of  moderate  size  is  always  convenient,  even 
where  dynamos  are  employed,  and  in  preparing  specifications 
for  a  machine,  its  use  as  a  battery  charger  should  be  kept  in 
mind  and  provided  for.  We  will  suppose  that  for  the  ordinary 
work  of  the  laboratory  a  current  of  25  amperes  and  10  volts 
is  required,  but  that  a  battery  of  eight  elements  with  normal 
charging  rate  of  25  amperes  is  to  be  maintained,  and  it  is 
desired  to  charge  them  all  simultaneously,  i.e.  in  series,  which 
would  call  for  a  current  pressure  of  20  volts.  In  this  case  a 
dynamo  with  an  output  of  500  watts  and  wound  to  20  volts  and 
25  amperes  would  answer  both  purposes.  Used  as  a  charger, 
the  maximum  voltage  of  the  dynamo  would  be  utilized,  while 
at  other  times  it  would  be  cut  down  to  one-half — with  little 
waste  of  energy  —  by  throwing  resistance  into  the  field. 

An  automatic  cut-out  should  be  inserted  in  the  line  from  the 
dynamo  to  the  battery  to  break  the  circuit  and  thus  prevent  a 
return  of  the  battery  current  through  the  armature  whenever 


482 


QUANTITATIVE  EXERCISES 


the  dynamo,  from  any  cause,  fails  to  generate.     All  circuits, 
moreover,  should  be  provided  with  safety  fuses. 

Another  form  of  machine  which  may  be  used  with  advantage 
wherever  the  work  to  be  done  is  continuous  and  varies  but  little 
in  quantity  is  the  so-called  dynamotor,  Fig.  68.  Dynamo  tors 
differ  from  motordynamos  in  that  the  motor  and  dynamo  arma- 
tures are  wound  together  upon  the  same  core,  so  that  a  single 
field  suffices  for  both  motor  and  dynamo.  The  voltage  of  the 
current  generated  by  these  machines  is  constant  and  cannot  be 
economically  regulated,  as  in  motordynamos,  by  changing  the 
resistance  in  the  field.  They  are  much  used  as  battery  chargers, 

electroplaters,  tele- 
phone ringers,  and  in 
telegraphy. 

For  work  through 
considerable  resist- 
ances, e.g.  50, 100,  or 
200  ohms,  dynamos  of 
relatively  large  elec- 
tromotive force  and 
moderate  current 
should  be  used.  An 
example  will  illus- 
trate the  advantage  of  such  a  machine  over  one  with  small 
electromotive  force  and  large  current  when  the  resistance  to  be 
overcome  is  high.  Suppose  the  resistance  of  a  circuit  is  100  ohms. 
A  dynamo  with  an  output  of  5  00  watts,  if  wound  for  20  volts  and 
25  amperes,  would  send  a  current  of  -f-^  =  0.2  ampere  through 
the  circuit ;  while  another  machine  with  the  same  output  and 
consuming  the  same  amount  of  energy  in  its  motor,  if  wound 
for  100  volts  and  5  amperes,  would  give  l-g-g-  =  1.0  ampere,  i.e. 
five  times  as  much  current.  There  is  also  a  great  waste  of 
energy  in  using  dynamos  of  high  electromotive  force  when  the 
resistance  to  be  overcome  is  small,  for  then  additional  resistance 
must  be  inserted  in  the  circuit  to  prevent  the  machine  from 


FIG.  68 


ELECTROLYTIC  DETERMINATION  OF  METALS        483 

generating  more  than  its  normal  maximum  of  current.  It  is 
well,  therefore,  to  have  two  motordynamos  in  a  laboratory,  one 
of  low  electromotive  force  and  large  current  for  work  involv- 
ing small  resistance,  such  as  the  precipitation  and  separation  of 
metals,  and  another  of  comparatively  large  electromotive  force 
for  work  in  which  higher  resistance  is  to  be  dealt  with. 

It  is  frequently  necessary  to  take  a  current  of  low  electro- 
motive force  from  one  of  high  pressure.  This  may  be  accom- 
plished by  inserting  in  the  circuit  a  lamp;  or  two  or  more  lamps 
in  parallel,  and  a  rheostat,  and  regulating  the  rheostat  until  the 
required  current  is  obtained.  Or  a  rheostat  may  be  inserted  in 
the  line  and  a  portion  of  its  resistance  short-circuited  by  means 
of  a  shunt.  The  voltage  of  the  shunted  current  will  equal  the 
fall  in  potential  between  the  two  points  where  the  shunt  joins 
the  main  line.  It  should  be  remembered,  however,  that  the 
cutting  down  of  pressure  by  these  methods  is  attended  by  a 
great  waste  of  energy. 

The  currents  obtained  from  dynamos  are  usiially  somewhat 
unsteady  owing  to  fluctuations  in  the  speed  of  the  machines. 
Hence,  when  currents  of  great  constancy  are  required,  storage 
batteries,  rather  than  dynamos,  should  be  employed. 

STORAGE  BATTERIES 

The  storage  battery  (accumulator,  secondary  or  reversible 
battery)  is  a  device  for  the  conversion  of  electrical  into  chemi- 
cal energy  which  may  be  reconverted  into  electrical  energy 
when  needed.  It  consists  essentially  of  two  series  of  parallel 
and  rigidly  connected  lead  plates  immersed  in  sulphuric  acid  of 
1.2  specific  gravity.  The  plates  of  one  series,  taken  together, 
constitute  the  positive  electrode,  while  those  of  the  other  series 
constitute  the  negative  electrode.  They  are  so  spaced  and 
arranged  in  the  liquid  that  every  positive  plate  lies  between 
two  negative  ones,  there  being  usually  one  more  of  the  latter 
than  of  the  former.  The  alternating  negative  and  positive 


484  QUANTITATIVE  EXERCISES 

plates  are  kept  from  accidental  contact  and  therefore  from  short 
circuits  within  the  cell  by  inserting  between  them  at  intervals 
narrow  strips  of  hard  rubber  or  of  some  other  nonconducting 
material.  Owing  to  the  large  surface  of  the  plates,  the  internal 
resistance  of  the  battery  is  exceedingly  small. 

If  the  two  series  of  plates  are  covered  with  lead  sulphate  and 
suspended  in  dilute  sulphuric  acid,  and  a  current  from  an  exter- 
nal source  is  passed  through  the  cell,  the  lead  of  the  sulphate 
is  converted  on  the  plates  belonging  to  the  pole  by  which  the 
current  enters  into  peroxide,  while  on  the  plate  belonging  to  the 
pole  by  which  the  current  leaves  the  cell  it  is  deposited  as  a 
spongy  metal.  The  two  series  of  plates  thus  acquire  different 
and  characteristic  colors,  —  the  first  a  dark  brown  and  the  second 
a  light  slate  color.  The  brown  or  oxide-covered  plates,  together 
with  the  heavy  lead  bar  to  which  they  are  joined,  make  up  the 
positive  electrode,  while  the  negative  electrode  consists  of  the 
slate-colored  plates.  The  so-called  "  active "  material,  which 
before  charging  is  lead  sulphate  and  after  charging  is  lead  per- 
oxide on  the  positive  and  metallic  lead  on  the  negative  electrode, 
is  originally  deposited  upon  and  within  the  plates  by  various 
patented  processes. 

If  the  two  poles  of  a  charged  cell  are  connected,  the  spongy 
lead  on  the  negative  plates  is  converted  into  sulphate,  and 
simultaneously  an  equivalent  amount  of  peroxide  on  the  posi- 
tive plates  is  reduced  by  hydrogen  to  the  lower  oxide,  which 
is  then  converted  also  into  sulphate.  Meanwhile  a  current 
passes  from  the  negative  to  the  positive  plates  within,  and  from 
the  latter  back  to  the  former  without  the  cell.  The  following 
tentative  explanation  has  been  given  for  the  conduct  of  the 
battery  during  the  discharging  and  charging  periods.  When 
charged,  i.e.  when .  the  negative  plates  are  properly  supplied 
with  spongy  lead  and  the  positive  plates  with  peroxide,  the 
liquid  contains  sulphuric  acid  and  lead  sulphate  in  the  electro- 
lytically  dissociated  condition.  In  the  vicinity  of  the  positive 
plates  there  is  probably  also  some  dissociated  peroxide,  that  is, 


ELECTROLYTIC  DETERMINATION  OF  METALS        485 

quadrivalent  lead  ions  together  with  an  equivalent  number  of 
hydroxyl  ions.  Suppose  now  the  poles  are  joined  and  that  an 
atom  of  spong}*-  lead  at  the  negative  pole  is  dissolved,  i.e.  passes 
into  the  liquid  carrying  its  two  charges  of  positive  electricity. 
The  consequences  would  be  twofold.  In  the  first  place,  the 
pole  would  be  negatively  charged ;  secondly,  two  hydrogen  ions 
belonging  to  the  dissociated  sulphuric  acid  would  be  compelled 
to  leave  the  solution.  This  they  may  do  by  uniting  at  the 
positive  pole  with  two  hydroxyl  ions  belonging  to  the  dissoci- 
ated peroxide.  The  lead  ion  would  thus  be  reduced  from  the 
quadrivalent  to  the  bivalent  condition,  giving  up  simultaneously 
two  of  its  four  positive  charges  of  electricity  to  the  adjacent  pole. 
In  this  way  the  two  poles  would  become  oppositely  charged 
to  the  same  degree.  The  bivalent  lead  would,  of  course,  react 
with  sulphuric  acid,  forming  sulphate  and  water. 

Suppose,  when  the  battery  is  in  the  discharged  condition,  i.e. 
when  the  plates  of  both  poles  are  covered  with  lead  sulphate 
and  the  liquid  is  saturated  with  the  same  compound,  that  a 
current  from  an  external  source  is  passed  into  it  at  the  posi- 
tive pole,  and  that  a  lead  ion  carrying  two  charges  of  elec- 
tricity receives  two  more  and  separates  from  the  solution  with 
four  hydroxyl  ions  derived  from  dissociated  water.  The  liquid 
would  now  contain  too  many  hydrogen  ions  by  two,  and  this 
would  compel  the  deposition  of  a  molecule  of  hydrogen  or  an 
equivalent  amount  of  lead  upon  the  negative  pole.  As  a  mat- 
ter of  fact,  the  lead  is  deposited. 

The  acid  which  is  used  in  a  storage  battery  should  be  free 
from  metals  other  than  lead,  from  arsenic,  and  from  hydrochlo- 
ric and  nitric  acids.  Its  concentration  should  be  maintained  by 
adding  distilled  water  from  time  to  time  to  replace  the  water 
which  is  lost  by  evaporation.  When  set  up  for  the  first  time, 
the  cells  are  in  the  discharged  condition ;  that  is,  most  of  the 
lead  which  takes  part  in  the  chemical  reactions  is  in  the  form 
of  sulphate.  The  charging  should  begin  immediately  after 
placing  the  electrodes  in  the  acid  and  should  be  continued  at 


486  QUANTITATIVE  EXERCISES 

"  normal  rate  "  with  the  least  possible  interruption  until  it  is  fin- 
ished. It  takes  much  longer  to  charge  a  battery  for  the  first  time 
than  for  any  subsequent  recharging.  For  example,  a  battery 
whose  normal  rate  is  12.5  amperes  for  8  hours  will  require  the 
same  current  for  nearly  30  hours  when  it  is  first  set  up.  Though 
the  first  charge  should  be  at  normal  rate,  that  is,  at  the  maximum 
rate  suitable  for  the  particular  type  of  cell,  the  battery  may  be 
recharged  with  smaller  currents.  The  time  required,  however, 
will  be  proportionately  increased.  If  a  battery  may  be  charged 
in  8  hours  with  a  current  of  12.5  amperes,  it  will  require 
8  x  12.5=100  hours  to  charge  it  with  one  ampere. 

When  charged  for  the  first  time,  it  is  recommended  by  the 
manufacturers  to  discharge  the  battery  about  one-half  and  then 
to  recharge  it  immediately,  and  to  repeat  this  treatment  two  or 
three  times,  or  until  the  color  of  the  positive  plates  is  a  deep 
brown  and  that  of  the  negative  plates  a  light  slate.  The  volt- 
age of  the  charging  current  should  only  slightly  exceed  the 
counter  electromotive  force  of  the  battery. 

There  are  certain  facts  connected  with  the  charging  of  a 
battery  which  require  attention  in  order  that  it  may  be  known 
whether  the  process  is  advancing  normally  and  when  it  is  to  be 
discontinued: 

1.  In  discharging,  the  voltage  of  a  cell  is  not  allowed  to  fall 
below  1.8.  On  recharging,  the  electromotive  force  of  the  cell 
rises  quickly  to  perhaps  2.35  volts,  where  it  remains  during  the 
larger  portion  of  the  charging  period.  Later  it  rises  quite 
rapidly  to  2.5  volts.  At  this  point  the  charging  should  be  dis- 
continued. If  cells  which  have  been  discharged  to  different 
degrees  are  in  series,  some  of  them  will  be  filled  before  others. 
In  this  case  the  charging  current  should  not  be  allowed  to  flow 
through  the  whole  series  until  all  the  cells  are  full.  Each  cell 
should  be  cut  out  as  soon  as  its  electromotive  force  reaches  2.5 
volts.  This  may  be  done,  but  the  voltage  of  the  charging  cur- 
rent should  be  correspondingly  diminished,  or  a  resistance  equal 
to  that  of  the  cells  retired  should  be  inserted  in  the  circuit. 


ELECTROLYTIC  DETERMINATION  OF  METALS        487 

2.  When  the  charging  of  a  cell  is  nearly  finished,  i.e.  when 
most  of  the  lead  of  the  sulphate  produced  in  discharging  has 
been  reconverted  into  peroxide  at  the  positive  pole  or  deposited 
as  metal  at  the  other  electrode,  it  begins  to  "  gas  "  because  of 
the  liberation  of  oxygen  at  the  positive  and  of  hydrogen  at  the 
negative  pole  ;  and  finally  the  evolution  of  gas  becomes  so  rapid 
as  to  give  the  liquid  the  appearance  of  boiling.     At  this  stage 
the  cell  should  have  an  electromotive  force  of  2.5  volts,  and  the 
negative  plates  should  exhibit  the  characteristic  slate  color.     If 
a  cell  fails  to  "  gas "  after  a  reasonable   period,  it  should  be 
examined  for  a  short  circuit,  which  may  occur  in  consequence 
of  a  warping  of  the  plates  or  of  the  presence  of  conducting 
material  between  them.     In  prying  the  plates  apart  or  dislodg- 
ing material  which  has  fallen  between  them,  a  rod  of  hard  rub- 
ber or  of  wood  —  and  never  a  metallic  one  —  should  be  used. 

3.  During  the  discharge  of  a  battery  much  of  the  sulphuric 
acid  is  converted  into  lead  sulphate,  which,  owing  to  its  sparing 
solubility,  is   mostly  deposited  upon  the  plates.     The   conse- 
quence  of  this  disappearance  of  acid  from  the  solution  is  a 
lowering  of  the  specific  gravity  of  the  liquid,  which  is  lightest, 
of  course,  when  the  battery  is  fully  discharged.    On  recharging, 
the  acid  is  regenerated  and  the  specific  gravity  of  the  liquid 
increases,  reaching  its  maximum  when  there  is  no  more  sulphate 
to  be  decomposed.     Hence,  if  care  is  taken  to  maintain  a  nearly 
constant  volume  of  liquid  in  a  cell  by  replacing  the  water  lost 
by  evaporation,  one  can  judge  quite  accurately  of  its  condition 
by  simply  taking  the  specific  gravity  of  the  liquid. 

4.  As  noted  already,  the  electromotive  force  of  a  cell  should 
not  be  allowed  to  fall  below  1.8  volts.     When  this  point  is 
reached,  it  should  be  recharged  with  the  least  possible  delay. 
If  the  discharge  has  been  carried  too  far,  or  if  a  cell  has  been 
allowed  to  stand  in  the  discharged  condition,  it  should  be  re- 
charged at  half  rate  and  until  the  potential  rises  to  2.4  volts. 

The  charged  cell  has  an  electromotive  force  of  2.5  volts  only 
while  the  charging  current  is  flowing.     On  discontinuing  the 


488  QUANTITATIVE  EXERCISES 

current,  the  potential  falls  to  2.2  volts,  and  soon  after  begin- 
ning to  discharge  it  sinks  to  about  2  volts.  From  this  point 
the  loss  of  electromotive  force  is  gradual. 

If  a  battery  is  to  be  "  put  out  of  commission  "  for  a  time,  the 
manufacturers  recommend  the  following  procedure :  It  is  first 
fully  charged.  The  acid  is  then  removed  by  means  of  a  siphon, 
and  the  jars  are  immediately  filled  with  water.  Lastly,  the  battery 
is  discharged  until  the  potential  sinks  below  one  volt  per  cell, 
when  the  water  is  withdrawn.  To  bring  the  battery  into  use 
again,  it  is  to  be  treated  as  if  it  were  receiving  its  first  charge. 


ELECTROLYSIS 
1.  THE  ELECTRODES 

A  simple  platinum  dish  with  a  capacity  of  200  or  250  cc. 
serves  well  as  a  cathode  for  the  deposition  of  most  metals.  In 
order  that  a  good  contact  between  the  dish  and  the  negative 
wire  of  the  circuit  may  be  secured,  a  heavy  brass  plate  is  pro- 
vided with  a  binding  screw  at  one  end  and  is  hollowed  out  in 
the  middle  to  fit  the  bottom  of  the  dish.  If  the  contents  of  the 
dish  are  to  be  warmed  during  an  electrolysis,  as  they  frequently 
must  be,  a  piece  of  asbestus  board  is  laid  upon  a  tripod  or  upon 
the  ring  of  an  iron  stand,  and  upon  this  are  placed  the  brass  plate 
and  the  dish.  A  small  flame  under  the  asbestus  easily  raises 
the  temperature  of  the  liquid  in  the  dish  to  50°  or  80°  without 
danger  that  the  boiling  point  will  be  reached. 

A  suitable  anode,  or  positive  electrode,  is  made  from  a  disk 
of  heavy  platinum  foil  50  mm.  in  diameter..  To  the  center  of 
this  is  attached  a  stout  platinum  wire  about  150  mm.  in  length. 
To  secure  a  good  contact  between  the  wire  and  the  disk  the 
former  is  flattened  at  one  end  and  bent  to  a  right  angle.  The 
disk  is  laid  on  a  hard,  smooth,  and  nonconducting  surface, — e.g. 
on  a  porcelain  block, — the  flattened  portion  of  the  wire  is  placed 
upon  the  center  of  the  disk,  and  the  parts  in  contact  are  heated 


ELECTROLYTIC  DETERMINATION  OF  METALS         489 

with  the  flame  of  the  blast  lamp.  When  quite  hot  the  parts  can 
be  firmly  welded  by  striking  with  a  small  hammer.  The  disk 
should  be  provided  with  a  considerable  number  of  small  holes  for 
the  escape  of  gas,  which  otherwise  would  accumulate  under  it 
and  interfere  with  the  electrolysis.  These  should  be  made  by 
inverting  the  disk  on  a  block  of  wood  and  punching  it  with  an 
awl.  If  the  holes  are  made  from  the  upper  side  of  the  disk,  gas 
will  collect  and  remain  under  the  electrode,  owing  to  the  down- 
ward direction  of  the  conical  elevations  produced  by  the  awl. 

The  dish  is  to  be  covered  during  an  electrolysis  with  a  watch 
glass  which  has  been  bored  in  the  center  to  accommodate  the 
standard  of  the  anode.  Such  holes  are  easily 
made  in  glass  by  boring  with  a  piece  of  tem- 
pered steel,  using  a  solution  of  camphor  in 
turpentine  as  a  lubricant.  An  efficient  bor- 
ing implement  for  the  purpose  may  be  made 
by  grinding  to  proper  shape,  and  then  tem- 
pering, the  soft  end  of  a  small  file.  The 
point  of  the  drill  should  be  quite  fine  and 
must  have  a  number  of  sharp  edges. 

A  great  variety  of  other  forms  have  been 
proposed  for  electrodes  and  some  of  these  are 
in  general  use  for  particular  determinations, 
such  as  that  of  copper,  but  the  platinum  dish 
and  disk  described  above  are  satisfactory  for 
nearly  every  kind  of  electrolytic  deposition. 

A  convenient  stand  for  electrodes  is  shown 

in  Fig.  69.  The  standard  on  which  are  mounted  the  brass  ring 
with  its  three  platinum  points  for  the  support  of  the  platinum 
dish  and  the  clamp  for  the  anode,  consists  of  a  stout  glass  rod. 

The  relation  of  the  area  of  an  electrode  used  as  cathode  to 
the  quantity  of  the  current  determines,  to  a  great  extent,  the 
character  of  the  metallic  deposits.  If  the  current  is  large  and  the 
electrode  small,  the  rapid  separation  of  metal  upon  the  limited 
surface  is  apt  to  give  poorly  adhering  and  even  spongy  deposits ; 


490  QUANTITATIVE  EXERCISES 

while  if  the  current  is  small  and  the  electrode  large,  the  deposits 
lack  uniformity  of  thickness  and  present  the  appearance  of  irreg- 
ular patches.  It  is  therefore  necessary  to  select  a  unit  of  elec- 
trode surface  and  to  define  the  density  of  the  current  with  respect 
to  it.  The  unit  chosen  is  100  square  centimeters,  which  is  des- 
ignated by  the  symbol  ND100.  If  the  current  is  1  ampere  and 
the  area  of  the  electrode  is  100  square  centimeters,  the  density 
is  said  to  be  1  ampere  with  respect  to  the  electrode  ;  while  if  the 
electrode  were  half  as  large,  —  50  square  centimeters  in  area, — 
the  density  would  be  2  amperes,  etc.  In  precipitating  metals, 
the  density  of  the  current  with  respect  to  the  cathode  is  of 
importance,  but  the  anode  may  be  of  any  convenient  size. 

2.  RHEOSTATS 

The  current  is  regulated  to  the  requirements  of  the  work  in 
hand  by  means  of  rheostats.  The  form  of  the  instrument  shown 
in  Fig.  TO  is  to  be  recommended.  These  rheostats  are  made 

with  any  range  of  re- 
sistance and  to  carry 
with  safety  any  quan- 
tity of  current  which 
the  purchaser  may  des- 
ignate. Three  small 
instruments,  with  ten 

steps  each,  when  con- 

FIG.  70  L  j    •          '•         '  -n 

nected   in   series,   will 

give  every  degree  of  resistance  from  0.1  ohm  to  111  ohms,  by 
tenths  of  a  unit,  if  the  resistances  of  the  three  are  respectively 
10  ohms,  1  ohm,  and  0.1  ohm  on  each  step.  In  other  words, 
with  such  an  arrangement,  1110  different  combinations  can  be. 
made.  It  is  well  to  construct  the  rheostat  of  highest  resistance 
with  more  than  10  steps,  since  each  additional  step  in  this 
instrument  makes  possible  a  hundred  new  combinations.  There 
is  no  subdivision  or  multiplication  of  the  resistance  that  is  likely 


ELECTROLYTIC   DETERMINATION    OF   METALS         491 

to  be  desired  which  cannot  be  effected  by  the  system.  For 
instance,  if  a  fourth  rheostat  with  0.01  ohm  resistance  on  each 
step  is  added  to  the  series,  the  number  of  possible  combinations 
rises  immediately  to  11,110.  It  should  be  borne  in  mind  in  pre- 
paring specifications  for  rheostats  that  the  higher  the  required 
resistance  the  finer  must  be  the  wire,  and  that  fine  wires  cannot 
carry  large  currents.  In  other  words,  it  is  impracticable,  as  it 
is  also  unnecessary,  to  combine  large  current-carrying  capacity 
with  high  resistance.  In  general,  only  rheostats  of  small  resist- 
ance are  required  to  carry  large  currents. 

3.  FARADAY'S  LAWS 

1.  When  a  current  is  passed  through  an  electrolyte,  the  quan- 
tities of  the  ions  which  separate  at  the  poles  are  proportional 
to  the  intensity  of  the  current.     For  example,  if  1  ampere  in 

1  second  (1  coulomb)  precipitates  0.0011172  gram  of  silver,  then 

2  amperes  will  precipitate  2  x  0.0011172  gram  in  the  same  time. 

2.  If  a  given  current  is  passed  through  any  number  of  elec- 
trolytes  arranged    in   series,   the    weights    of  the    ions    which 
separate  at  the  various  poles  are  related  to  each  other  as  their 
chemically    equivalent    weights.     If,    for    instance,    a   current 
passes  through  three  cells  in  series,  the  first  containing  a  solu- 
tion of  a  silver  salt,  the  second  a  salt  of  zinc,  and  the  third  a 
cupric  salt,  the  weights  of  the   metals  precipitated  upon  the 

f\A.  Q     fiQ  1 

three  cathodes  will  be  related  as  107.11 :  — ^-  :  —^-  . 

The  weights  of  the  ions  which  are  chemically  equivalent  are 
found,  of  course,  by  dividing  their  atomic  weights — or  the 
sum  of  the  weights  of  the  atoms  contained  in  them  if  they  are 
complex  —  by  their  respective  valencies. 

The  so-called  electro-chemical  equivalent  of  an  ion  is  the 
weight  of  it  which  is  separated  at  an  electrode  by  1  coulomb  of 
current, — that  is,  by  1  ampere  in  1  second.  It  may  be  readily 
found  in  any  case  (if  the  system  of  atomic  weights  based  on  the 


492  QUANTITATIVE  EXERCISES 

weight  of  hydrogen  as  unity  is  employed)  by  multiplying  the 
atomic  weight  of  the  ion  or  the  sum  of  the  weights  of  the  atoms 
in  it,  by  0.0000104304  (the  electro-chemical  equivalent  of  hydro- 
gen) and  dividing  by  the  valence  of  the  ion. 

It  is  to  be  noted  that  metals  which,  like  copper,  mercury, 
iron,  etc.,  form  more  than  one  series  of  salts  with  electro- 
negative elements  or  groups,  have  also  more  than  one  electro- 
chemical equivalent.  The  equivalent  of  copper,  for  example,  is 
0.0003292  gram  in  cupric  salts,  and  0.0006584  gram  in  cuprous 
salts.  In  ferric  salts  the  equivalent  of  iron  is  0.00019331  gram, 
and  in  ferrous  salts  0.00028998  gram. 

To  separate  the  number  of  grams  of  any  ion  which  is  equal 
to  its  chemically  equivalent  weight  —  e.g.  1  gram  of  hydrogen, 
35.18  grams  of  chlorine,  107.11  grams  of  silver,  47.6T5  grams 
SO4,  etc.  —  95,874  coulombs  are  required. 

To  find  what  weight  of  any  ion  will  be  separated  in  a  given 
time  by  a  given  current,  it  is  necessary  only  to  reduce  the 
current  and  time  to  coulombs  and  to  multiply  by  the  electro- 
chemical equivalent  of  the  ion. 

To  find  what  current  will  be  required  to  separate  a  given 
weight  of  any  ion  in  a  fixed  time,  the  weight  is  divided  by  the 
electro-chemical  equivalent  of  the  ion,  and  the  quotient  by  the 
time  in  seconds.  The  second  quotient  will  be  the  number  of 
amperes  which  must  be  employed. 

A  necessary  conclusion  from  the  facts  embraced  by  Faraday's 
law  is  that  ions  of  equal  valence  carry  equal  charges  of  electricity, 
positive  or  negative  ;  also  that  bivalent  ions  carry  twice  as  much 
as  univalent  ions,  etc., — in  other  words,  that  the  magnitude  of  the 
charge  carried  by  an  ion  is  strictly  proportional  to  its  valence. 

The  current  enters  a  solution  at  the  anode  or  positive  elec- 
trode, and  leaves  it  at  the  cathode  or  negative  electrode.  The 
ions  which  move  toward  the  anode  are  negatively  charged  and 
are  called  anions,  while  those  which  move  toward  the  cathode, 
i.e.  in  the  direction  of  the  current,  are  positively  charged  and 
are  called  cations. 


ELECTROLYTIC  DETERMINATION  OF  METALS        493 

4.  POLARIZATION  CURRENTS 

The  metal  which  is  deposited  upon  the  cathode  during  the 
progress  of  an  electrolytic  separation  tends  to  return  to  the  ionic 
condition  owing  to  its  so-called  electrolytic  solution  pressure. 
The  same  phenomenon  presents  itself  whenever  any  separa- 
tion takes  place  upon  insoluble  electrodes,  whether  the  depos- 
its are  solids  or  gases.  The  result  is  an  electromotive  force  or 
"  polarization  current  "  in  the  cell  opposed  to  that  which  is  pro- 
ducing the  deposition.  In  order,  therefore,  that  a  metal  may  be 
precipitated  or,  more  generally,  that  any  kind  of  electrolytic 
separation  may  be  effected,  a  current  must  be  employed  whose 
electromotive  force  is  superior  to  that  of  the  polarization  cur- 
rent. Otherwise  no  continuous  separation  will  take  place.  These 
statements  do  not  apply  to  cells  like  plating  baths,  in  which  the 
anodes  are  made  of  the  plating  metal  and  are  therefore  dissolved 
as  fast  as  the  metal  is  deposited  on  the  cathode.  In  such  cells 
there  is  no  polarization  current  in  the  usual  sense. 

Le  Blanc  has  determined  the  magnitudes  of  the  electromotive 
force  which  just  suffices  for  the  decomposition  of  a  number  of 
substances  in  normal  solutions.  The  following  values  were 
found  for  salts  from  which  the  metal  is  precipitated: 

ZnSO4        =  2.35  volts  Cd(NO3)2  =  1.98  volts 

ZnBr2-        =1.80      «  CdSO4        =2.03      « 

NiSO4        =2.09      «  CdCl2         =1.88      « 

NiCl2         =1.85      «  CoSO4        =1.92     « 

Pb(NO8)2  =  1.52      «  CoCl2         =1.78     « 
AgNO8      =  0.70  volt 

The  decomposition  values  of  the  common  acids  and  bases 
were : 

Acids 

Hydrochloric  =  1.31  volts  Hydrobromic  =  0.94  volt 

Nitric  =  1.69      "  Hydriodic        =  0.52    « 

Sulphuric        =  1.67      "  Oxalic  =  0.95    « 

Phosphoric      =1.70      " 


494  QUANTITATIVE  EXERCISES 

Bases 

Sodium  hydroxide          =  1.69  volts 
Potassium  hydroxide     =1.67      " 
Ammonium  hydroxide  =  1.74      " 

The  fact  that  each  metal  requires  for  its  precipitation  a  certain 
minimum  electromotive  force,  below  which  it  cannot  separate 
from  solutions  of  its  salts,  and  that  these  minima  are  different 
for  different  metals,  is  of  great  practical  importance,  since  it 
makes  possible  a  satisfactory  electrolytic  separation  of  several 
of  the  metals.  If,  for  example,  a  solution  contains  copper  and 
zinc  as  sulphates,  the  former  may  be  completely  precipitated  by 
a  current  whose  voltage  is  kept  above  1.22,  but  is  not  allowed 
to  rise  above  1.4,  while  all  of  the  zinc  remains  in  solution.  On 
raising  the  voltage  to  2.35  or  higher,  the  zinc  also  is  precipi- 
tated. 

The  pressure  of  current  which  is  required  for  the  electro- 
lytic decomposition  of  a  substance  may  be  calculated  from  the 
heat  of  its  formation.  For  example,  the  heat  of  formation 
of  an  electro-chemical  gram-equivalent  weight  of  zinc  sulphate 

|  -  -  =  80.13  grams  J  is  approximately  53,045  calories, 

while  the  equivalent  of  the  calorie  in  electrical  energy  is 
4.2  joules.  The  heat  of  formation  of  80.13  grams  of  zinc  sul- 
phate is,  therefore,  expressed  in  electrical  units,  equivalent 
to  53045  x  4.2,  or  222789  joules ;  and  ^sW'  or  2-82  volts>  is 
the  current  pressure  which  must  be  maintained  in  order  to  effect 
the  electrolysis  of  the  salt.  The  value  found  experimentally  by 
Le  Blanc  was  2.35  volts. 


ELECTKOLYTIC   DETMKMIN  ATION    OF   MKTALS         495 

EXERCISE  LVIII 
ELECTROLYTIC  DETERMINATIONS 

1.  COPPER 
a.   Classen  s  Method 

Dissolve  a  weighed  quantity  (0.2  to  0.5  gram)  of  pure  copper 
sulphate  in  a  little  water  and  add  an  excess  of  a  saturated  solu- 
tion of  ammonium  oxalate.  Heat  to  about  80°  and  electrolyze 
for  a  few  minutes ;  then  add  from  a  burette  with  a  fine  outlet, 
at  the  rate  of  about  10  drops  per  minute,  a  saturated  solution 
of  oxalic  acid,  allowing  the  acid  to  fall  upon  the  covering  glass, 
from  which  it  will  find  its  way  into  the  dish  through  the  hole 
in  the  center  of  the  glass. 

To  determine  when  the  precipitation  of  the  copper  is  finished, 
a  drop  of  the  liquid  is  taken  out  from  time  to  time  after  about 
two  hours,  and  tested  on  a  porcelain  surface  with  a  drop  of 
a  solution  of  potassium  ferrocyanide  which  has  been  acidified 
with  hydrochloric  acid. 

At  the  end  of  the  electrolysis  the  liquid  in  the  dish  is  to  be 
removed  and  replaced  by  water  without  interrupting  the  cur- 
rent. For  this  purpose  two  siphons  are  employed,  one  for  the 
removal  of  the  liquid  and  the  other  for  the  introduction  of 
water. 

Finally,  the  deposited  metal  is  washed  repeatedly  with  water 
and  then  with  small  quantities  of  alcohol,  and  dried  in  an  air 
bath  at  80°  to  90°. 

The  conditions  of  the  experiment  as  given  by  the  author  of 
the  method  are : 

Temperature  of  the  liquid,  80°. 

Density  of  the  current,  ND100  =  0.5  to  1.0  ampere  ;  most 

favorable  density,  1.0  ampere. 
Electrode  tension,  2.5  to  3.2  volts. 
Time,  2  hours. 


496  QUANTITATIVE  EXERCISES 

The  particular  advantage  claimed  for  the  method  is  the  short 
time  required  for  the  deposition  of  the  metal. 

In  the  case  of  quite  dilute  solutions  of  copper  salts  the  oxalic 
acid  may  be  introduced  from  the  beginning  of  the  electrolysis, 
whereas  in  concentrated  ones  such  a  course  would  result  in  the 
precipitation  of  a  difficultly  soluble  oxalate  of  copper. 

b.  Deposition  in  the  Presence  of  Nitric  Acid 

Dissolve  a  weighed  quantity  of  copper  sulphate  in  water,  add 
2  or  3  cc.  of  dilute  nitric  acid,  and  electrolyze. 
Conditions  of  the  experiment : 

Temperature,  20°  to  30°. 

Density  of  the  current,  ND100  =  0.5  to  1  ampere,  —  the 
last  only  when  no  other  metal  than  copper  is  present. 
Electrode  tension,  2.2  to  2.5  volts. 
Time,  5  to  6  hours. 

Chlorides  must  be  absent,  owing  to  the  tendency  of  copper  in 
their  presence  to  precipitate  in  spongy  form. 

Copper  can  be  separated  from  iron,  nickel,  cobalt,  manganese, 
zinc,  and  cadmium  in  the  manner  described  under  6,  but  the 
quantity  of  the  nitric  acid  must  then  be  increased  to  20  cc. 
(sp.  gr.  1.21).  It  cannot  be  separated  from  antimony,  arsenic, 
mercury,  silver,  tin,  and  bismuth. 

2.  NICKEL 
Classen's  Method 

Dissolve  a  weighed  quantity  of  ammonium  nickel  sulphate  in 
about  25  cc.  of  water,  warm,  add  from  6  to  8  grams  of  ammo- 
nium oxalate,  and  dilute  the  solution  to  100  or  120  cc.  During 
the  electrolysis  maintain  a  temperature  of  GO0  to  70°.  The  com- 
pletion of  the  deposition  is  tested  with  ammonium  sulphide. 


ELECTROLYTIC  DETERMINATION  OF  METALS        497 

Conditions  of  the  experiment : 

Temperature,  60°  to  70°. 
Density  of  current,  ND100,  =  1  ampere. 
Electrode  tension,  3.1  to  3.8  volts. 
Time,  3  to  5  hours. 

Cobalt  may  be  satisfactorily  determined  under  precisely  the 
same  conditions. 

Nickel  and  cobalt  may  also  be  determined  under  the  condi- 
tions recommended  by  Fresenius  and  Bergmann  as  follows: 
The  solution  of  the  salt  is  treated  with  from  4.5  to  6  grams  of 
ammonium  sulphate  and  40  cc.  of  ammonia  of  0.96  specific 
gravity,  and  diluted  to  150  or  170  cc.  If  more  than  0.5  gram 
of  the  metal  is  to  be  precipitated,  the  quantity  of  the  ammonia 
is  increased  to  50  or  60  cc. 

Conditions  of  the  experiment : 

Temperature,  that  of  the  room. 

Density  of  the  current,  ND100,  =  0.5  to  0.7  ampere. 

Electrode  tension,  2.8  to  3.3  volts. 

Chlorides,  nitrates,  fixed  organic  acids,  and  magnesium  com- 
pounds should  not  be  present. 

3.  IRON 

Classen's  Method 

Dissolve  from  6  to  8  grams  of  ammonium  oxalate  in  the  least 
possible  quantity  of  warm  water,  add  slowly  and  with  constant 
stirring  the  solution  of  the  iron  salt  ((NH4)2  FeSo4).  Dilute  to 
100  or  150  cc.  and  electrolyze. 

Conditions  of  the  experiment : 

Temperature,  20°  to  40°. 

Density  of  the  current,  ND100,  =  1  to  1.5  amperes. 

Electrode  tension,  3.6  to  4.3  volts. 

Time,  3  to  4  hours. 


498  QUANTITATIVE  EXERCISES 

At  the  close  of  the  precipitation  the  dish  is  quickly  emptied. 
The  deposit  is  washed  repeatedly  with  water  and  with  small 
quantities  of  pure  alcohol,  and  then  dried  at  a  temperature  of 
TO0  to  90°. 

The  best  compounds  from  which  to  precipitate  iron  in  the 
manner  described  above  are  the  double  ammonium  sulphates. 
If  the  solutions  to  be  electrolyzed  contain  chlorides  or  nitrates, 
they  should  be  evaporated  with  sulphuric  acid  until  all  hydro- 
chloric and  nitric  acids  have  been  expelled.  The  excess  of  the 
sulphuric  acid  is  afterwards  neutralized  with  ammonia. 


4.  ZINC 

Classen's  Method 

Owing  to  the  difficulty  of  removing  electrolytically  deposited 
zinc  from  platinum,  the  dish  used  as  cathode  should  first  be 
plated  on  the  inside  with  copper  or  silver.  The  former  metal 
may  be  deposited  in  the  manner  described  under  1,  a,  or  1,  b.  If 
the  dish  is  to  be  plated  with  silver,  the  metal  is  best  precipitated 
from  a  solution  of  its  double  salt  with  potassium  cyanide  by  a 
current  of  0:2  to  0.5  ampere. 

Having  prepared  the  dish,  dissolve  the  zinc  salt  in  the  smallest 
possible  quantity  of  warm  water,  and  add  about  4  grams  of 
ammonium  oxalate.  Heat,  adding  small  quantities  of  water  if 
necessary  until  all  of  the  salt  is  dissolved,  and  transfer  the  solu- 
tion to  the  dish.  Electrolyze  at  50°  to  60°  from  3  to  5  min- 
utes, arid  then  introduce  a  saturated  solution  of  oxalic  acid 
or  a  6  per  cent  solution  of  tartaric  acid,  as  directed  under  1,  a. 

Potassium  ferrocyanide  is  employed  to  determine  when  the 
precipitation  is  finished.  The  dish  must  be  washed  without 
interruption  of  the  current. 


ELECTROLYTIC  DETERMINATION  OF  METALS        499 

Conditions  of  the  experiment : 

Temperature,  50°  to  60°. 
Current  density,  ND100  =  0.5  to  1.0  ampere. 
Electrode  tension,  3.5  to  4.8  volts. 
Time,  2  hours. 


NOTE.  —  The  few  examples  given  above  will  serve  as  an  introduction 
to  analysis  by  electrolysis.  For  further  information  regarding  electrolytic 
determinations  and  separations,  the  student  is  recommended  to  consult 
Classen's  Quantitative  Analyse  durch  Electrolyse,  or  the  English  translation 
of  the  same  by  Herrick. 


CHAPTER  XXII 
BUTTER 

The  fats  and  oils,  whether  of  animal  or  of  vegetable  origin, 
are  principally  made  up  of  mixtures  of  the  neutral  glycerin 
salts  of  certain  organic  acids.  Of  these  acids  it  may  be  said, 
in  general,  that  they  are  all  mono-basic  and  contain,  in  most  in- 
stances, an  even  number  of  carbon  atoms  ;  also  that  they  belong, 
with  few  exceptions,  to  the  series  represented  by  the  formulas 
CwH2n02,  CMH2n_202,  CnH2n_402,  and  CttH2n_6O2.  The  following 
table  gives  the  composition  of  the  more  important  acids,  together 
with  some  examples  of  the  fats  in  which  their  occurrence  is 
characteristic. 

ACETIC  ACID  SERIES 

CKH2,02 
Acetic,  C2H4O2  Rare. 

Butyric,  C4H8O2  In  butter,  of  which  about  6  per  cent  consists  of  buty- 

rin,  the  neutral  glycerin  salt. 

Isovaleric,  C5H10O2  In  porpoise  and  dolphin  oils. 

Caproic,  C6H12O2  In  butter  and  cocoanut  oil. 

Caprylic,  C8H16O2  In  butter,  human  fat,  and  cocoanut  oil. 

Capric,  C10H20O2  In  butter  and  cocoanut  oil. 

Laurie,  C12H24O2  In  laurel  oil,  cocoanut  oil,  spermaceti,  etc. 

Myristic,  C14H28O2  In  nutmeg  butter,  Dika  oil,  and  cocoanut  oil. 

Palmitic,  C16H32O2  In  most  animal  and  vegetable  fats  and  oils. 

Stearic,  C18H36O2  In  most  fats. 

Arachidic,  C20H40O2  In  arachis  oil. 

Behenic,  C22H44O2  In  oil  of  behen. 

ACRYLIC  ACID  SERIES 


Tiglic,  C5H8O2  In  croton  oil. 

Oleic,  C18H34Og  In  most  fats  and  oils. 

Erucic,  C2,,H42O2         In  rape  oil. 

500 


BUTTER  501 

LINOLIC  ACID  SEKIES 

CnII2n_402 
Linolic,  C18H3202        In  linseed  and  other  drying  oils. 

LINOLENIC  ACID  SERIES 
CnH2n_6O2 

Linolenic,  C18H30O2     In  linseed  and  other  drying  oils. 

The  chemical  methods  which  are  employed  to  distinguish 
genuine  butter  from  the  various  manufactured  substances 
resembling  it  (oleomargarine,  butterine,  etc.),  and  to  determine 
whether  butter  has  been  adulterated  with  other  fats  or  oils,  are 
all  based  on  a  single  characteristic  difference  in  composition 
between  butter,  on  the  one  hand,  and  all  those  fats,  on  the 
other,  which  can  be  successfully  employed  to  counterfeit  or 
adulterate  it.  This  difference  lies  in  the  fact  that  butter  con- 
tains a  considerable  proportion  of  the  glycerin  salts  of  acids 
of  comparatively  small  molecular  weight,  —  butyric,  caproic, 
caprylic,  and  capric, —  while  the  other  fats,  with  possibly  one 
exception,  are  nearly  destitute  of  them. 

These  acids  of  low  molecular  weight  are  soluble  in  water  and 
can  therefore  be  readily  separated  from  those  of  high  molecular 
weight,  like  palmitic,  stearic,  and  oleic  acids,  which  are  insolu- 
ble in  water.  On  this  difference  is  based  the  process  of  Hehner, 
to  whom  belongs  the  credit  of  having  been  the  first  to  propose 
a  rational  method  for  distinguishing  butter  from  its  admixtures 
with  other  fats,  and  from  oleomargarine. 

They  are  also  volatile  with  water  vapor,  while  the  other  acids 
occurring  in  fats  and  oils  are  either  nonvolatile,  or  only  slightly 
volatile  with  steam.  This  is  the  basis  of  Reichert's  method. 

Again,  since  butter  contains  a  larger  proportion  of  acids  of 
small  molecular  weight,  it  follows  that  a  fixed  weight  of  it  requires 
more  alkali  for  its  saponification  than  the  same  weight  of  other 
fats.  On  this  fact  is  founded  the  method  of  Koettstorfer. 


502  QUANTITATIVE  EXERCISES 

EXERCISE   LIX 

DETERMINATION  OF  THE  INSOLUBLE  ACIDS 
HEHNER'S  METHOD 

Melt  about  100  grams  of  butter  in  a  beaker  glass,  taking  care 
not  to  raise  the  temperature  unnecessarily,  and  filter  the  upper 
oily  portion  through  a  dry,  warm  paper  into  a  large  test  tube. 
Place  the  tube  in  water  having  a  temperature  of  50°  or  60°,  and 
keep  it  there  until  the  oil  appears  to  be  free  from  water.  Filter 
through  a  dry,  warm  paper  into  large  weighing  glasses,  taking 
care  not  to  allow  any  water  which  may  have  separated  from  the 
butter  to  come  upon  the  filter.  Stir  the  filtrate  while  it  is  solidi- 
fying in  order  to  prevent  any  separation  of  the  constituents. 

Place  a  filter,  of  double  thick  paper  and  about  100  mm.  wide, 
in  a  weighing  glass  and  dry  it  to  constant  weight  at  100°. 

Weigh  about  2  grams  of  the  butter  into  a  beaker  glass  and 
add  to  it  50  cc.  of  85  per  cent  alcohol  and  a  piece  of  pure  potas- 
sium hydroxide  weighing  from  1  to  2  grams.  Heat  the  beaker 
gently  upon  a  water  bath  from  20  to  30  minutes.  Add  water, 
drop  by  drop,  stirring  the  liquid  after  each  addition.  If  it 
remains  clear,  the  saponification  is  complete.  If  it  becomes 
turbid,  the  addition  of  water  must  be  stopped  and  the  heating 
continued.  Evaporate  the  alcohol  and  dissolve  the  residue  in 
about  150  cc.  of  water.  Add  to  the  clear  solution  a  moderate 
excess  of  dilute  hydrochloric  acid.  Heat  until  the  insoluble 
acids  collect  in  liquid  form  at  the  top  and  the  solution  beneath 
becomes  quite  clear.  Place  the  weighed  filter  in  a  funnel,  fitting 
it  tightly  to  the  glass.  Pour  boiling  water  through  it,  and, 
while  it  is  still  half  full  of  water,  begin  to  filter  the  contents  of 
the  beaker.  Cleanse  the  beaker  thoroughly  with  the  boiling 
water,  and  wash  the  acids  on  the  filter  with  not  less  than  1| 
liters  of  boiling  water.  Place  the  filter,  while  it  is  still  some- 
what damp,  in  the  weighing  glass  in  which  it  was  previously 
dried,  and  heat  to  very  nearly  constant  weight  at  100°. 


BUTTER  503 

The  insoluble  acids,  according  to  Hehner,  the  author  of  this 
method,  constitute  from  86.5  to  87.5  per  cent  of  genuine  butter, 
though  in  some  cases  the  percentage  rises  to  88.  In  other  fats 
and  oils  the  percentage  of  insoluble  acids  is,  as  a  rule,  about 
95.5.  He  would  therefore  regard  as  spurious  any  specimen  of 
reputed  butter  which  is  found  to  contain  more  than  88  per  cent 
of  these  acids.  To  calculate  the  extent  of  the  adulteration, 
where  a  higher  per  cent  has  been  found,  he  assumes  that  87.5 
is  the  normal  percentage  of  insoluble  acids  in  butter,  and  finds 
the  proportion  of  the  foreign  fats  by  means  of  the  difference 
between  that  number  and  the  higher  per  cent  which  the  sample 
contains.  Accordingly,  a  specimen  which  yields  91  per  cent  of 
insoluble  acids  would  be  regarded  as  containing  43.75  per  cent 
of  foreign  fats,  since  95.5  -  87.5  =  8,  and  91.0  -  87.5  =  3.5, 
and  8:  3.5::  100:  43.75. 

The  conclusions  of  Hehner  regarding  the  quantity  of  insol- 
uble acids  which  is  normal  to  butter  have  been  repeatedly  veri- 
fied, but  not  by  all  observers.  Some  have  found  89  and  even 
over  90  per  cent  of  insoluble  acids  in  butter  whose  genuineness 
could  not  be  questioned. 

The  following  table  gives  the  percentage  of  insoluble  acids 
which  several  of  the  fats  and  oils  have  been  found  to  contain. 

Butter 87.50  Olive  oil 95.43 

Lard 96.15  Palm  oil 95.60 

Tallow 95.50  Peanut  oil    ....  95.00 

Oleomargarine .     .     .  95.56  Rape  oil 95.10 

'Cocoanut  oil      .     .     .  86.43  Sesame  oil    ....  95.48 

Cotton-seed  oil .     .     .  95.75 

A  portion  of  the  soluble  acids  of  the  butter  is  lost  during  sapon- 
ification  in  consequence  of  the  formation  of  volatile  ethyl  salts. 

If  the  liquid  under  the  layer  of  insoluble  acids  is  turbid  at 
the  time  of  filtration,  the  filtrate  will  also  be  cloudy  from  the 
presence  in  it  of  insoluble  acids  in  a  finely  divided  condition. 
It  is  therefore  important  to  postpone  the  filtration  until  the 
liquid  becomes  quite  clear.  A  slight  turbidity  can  be  removed 


504  QUANTITATIVE  EXERCISES 

by  passing  the  filtrate  repeatedly  through  the  filter.  The  fil- 
trate should  be  carefully  examined,  not  only  with  respect  to  its 
clearness,  but  also  for  the  presence  in  it  of  minute  globules  of 
the  insoluble  acids. 

The  washing  of  the  insoluble  acids  is  the  most  critical  feature 
of  the  determination.  In  the  liquid  condition  they  dissolve  con- 
siderable quantities  of  the  soluble  acids  and  retain  them  with 
great  tenacity.  The  water  should  be  forced  into  tne  funnel  in 
such  a  way  as  to  break  up  the  layer  of  insoluble  acids  to  the 
greatest  possible  extent.  In  this  manner  a  much  larger  surface 
is  brought  into  contact  with  the  water,  and  the  removal  of  the 
soluble  acids  is  correspondingly  more  rapid.  The  water  in  the 
funnel  should  be  allowed  to  drain  out  quite  completely  before 
introducing  a  fresh  portion.  A  very  faintly  alkaline  solution 
of  phenolphthalein  may  be  used  to  test  the  reaction  of  the 
filtrate. 

The  insoluble  acids  cannot  be  dried  to  a  perfectly  constant 
weight  in  the  air.  According  to  Hehner  they  lose  for  a  time 
and  then  gain  in  weight  in  consequence  of  oxidation.  How- 
ever, a  fairly  satisfactory  approach  to  constancy  may  be  obtained 
if  the  soluble  acids  have  been  completely  removed  by  the  wash- 
ing process. 

The  weight  of  the  insoluble  acids  may  be  obtained  by  placing 
the  filter,  after  drying  it  in  a  current  of  air,  in  a  Soxlet  appa- 
ratus and  extracting  it  with  ether.  The  ether  is  then  distilled 
off  and  the  residue  heated  to  constant  weight  at  100°. 

EXERCISE  LX 

DETERMINATION  OF  THE  VOLATILE  ACIDS 
REICHERT'S  METHOD 

Weigh  very  nearly  5  grams  of  the  butter,  prepared  as  directed 
under  LIX,  into  a  quarter-liter  Erlenmeyer  flask.  Add  to  it 
10  cc.  of  95  per  cent  alcohol  which  has  been  treated  with  a 


BUTTER  505 

little  sodium  hydroxide  and  then  redistilled,  and  2  cc.  of  a 
caustic  soda  solution  made  by  dissolving  100  grams  of  sodium 
hydroxide  in  100  cc.  of  water.  Attach  the  flask  to  an  inverted 
condenser  and  heat  for  one  hour  in  a  water  bath,  immersing  the 
flask  nearly  to  the  neck  in  the  boiling  water.  Evaporate  the 
alcohol  and  dissolve  the  residue  in  135  cc.  of  recently  boiled 
water.  To  the  clear  soap  solution,  when  its  temperature  has 
fallen  to  60°  or  70°,  add  5  cc.  of  dilute  sulphuric  acid,  prepared 
by  diluting  200  cc.  of  the  strongest  acid  to  one  liter.  Attach 
the  flask  to  the  inverted  condenser  and  heat  until  the  insoluble 
acids  collect  in  a  clear  oily  layer  at  the  top.  Allow  the  con- 
tents of  the  flask  to  cool  to  the  temperature  of  the  room.  Intro- 
duce a  few  pieces  of  pumice  stone  which  have  been  thrown 
into  water  while  at  a  white  heat  and  kept  there  until  needed. 
Attach  the  flask  to  a  condenser  arranged  for  distillation  and 
distill  off  110  cc.  in  30  minutes  as  nearly  as  possible.  Receive 
the  distillate  in  a  narrow  graduated  cylinder  so  that  the  rate 
of  the  distillation  may  be  properly  regulated.  Filter  the  dis- 
tillate, after  shaking  it  well,  through  a  dry  paper.  Titrate 
100  cc.  of  the  filtrate  with  a  tenth-normal  solution,  either  of 
barium  hydroxide  in  water  or  of  potassium  hydroxide  in  alco- 
hol, using  an  alcoholic  solution  of  phenolphthalein  as  the  indi- 
cator. Add  one-tenth  for  the  portion  of  acid  not  titrated. 

According  to  Reichert  and  many  other  chemists  who  have 
tested  his  method  with  care,  the  distillate  from  2.5  grams  of 
genuine  butter  neutralizes  from  13  to  15  cc.  of  tenth-normal 
alkali ;  while  that  from  an  equal  weight  of  any  other  fat  or  oil 
which  could  be  successfully  employed  to  adulterate  or  imitate 
butter  neutralizes,  generally,  less  than  2  cc.  Possibly  an  excep- 
tion should  be  made  in  the  case  of  cocoanut  oil,  whose  distillate 
has  been  found  to  neutralize  from  3.5  to  3.7  cc.  of  tenth-normal 
alkali.  Reichert  would  regard  as  certainly  adulterated,  or  as 
consisting  of  other  fats  than  butter,  any  sample  whose  volatile 
acids  neutralize  less  than  12.5  cc.  of  alkali.  The  following 
table  gives,  in  cubic  centimeters,  the  quantity  of  tenth-normal 


506  QUANTITATIVE  EXERCISES 

alkali  required  to  neutralize  the  volatile  acids  derived  from  2.5 
grams  of  some  of  the  fats  and  oils. 

Butter  ....  13.0  to  15.0  Cocoanut  oil .     .     .     .  3.70 

Lard 0.30  Palm-nut  oil  .     .     .     .  2.40 

Tallow 0.25  Palm  oil 0.80 

Oleomargarine    .     .     .     0.95  Rape  oil 0.25 

Cotton-seed  oil  .     .     .     0.30  Sesame  oil      ....  2.20 

Only  about  four-fifths  of  the  volatile  acids  in  the  butter  are 
found  in  the  distillate,  and  the  quantity  which  a  distillate  of 
given  volume  will  contain  varies  somewhat  with  the  volume  of 
the  liquid  distilled  and  the  rapidity  of  the  distillation.  Hence 
the  necessity  of  following  fixed  rules  as  to  the  volume  of  the 
liquid  distilled,  the  volume  of  the  distillate  collected  for  exam- 
ination, and  the  time  within  which  the  distillation  is  made. 

It  is  preferred  by  some  to  effect  the  saponification  in  a  strong 
flask,  which  is  closed  during  the  reaction  by  a  cork  securely 
tied  to  the  neck. 


EXERCISE  LXI 

DETERMINATION  OF   THE  ALKALI   REQUIRED   FOR 
SAPONIFICATION 

KOETTSTORFER'S  METHOD 

Dissolve  about  10  grams  of  potassium  hydroxide  in  2  liters 
of  95  per  cent  alcohol  and  redistill.  Dissolve  about  15  grams 
of  the  purest  potassium  hydroxide  in  400  or  500  cc.  of  the 
redistilled  alcohol  and  dilute  with  the  same  to  one  liter.  Pour 
the  solution  into  a  bottle  and  allow  it  to  stand  undisturbed  until 
the  potassium  carbonate  which  was  in  the  hydroxide  settles  out 
and  becomes  firmly  attached  to  the  glass.  - 

Prepare  a  standard  solution  of  hydrochloric  acid,  each  cubic 
centimeter  of  which  contains  the  quantity  of  acid  required  to 
neutralize  11.14  milligrams  of  potassium  hydroxide.  Weigh 


BUTTER  507 

from  1  to  2  grains  of  butter  into  a  small  Erlenmeyer  flask  and 
add  to  it  the  volume  of  the  alcoholic  potash  solution  which 
has  been  found  to  be  equivalent  to  40  cc.  of  the  standard  acid. 
Attach  the  flask  to  an  inverted  condenser  and  heat  it  on  a  water 
bath  until  the  saponification  is  complete.  Allow  the  flask  to 
cool  to  the  temperature  of  the  room  before  removing  it  from 
the  condenser.'  Add  an  alcoholic  solution  of  phenolphthalein 
and  titrate  the  excess  of  alkali  with  the  standard  acid. 

Koettstorfer  found  that  one-gram  quantities  of  genuine  butter 
from  different  sources  required  for  saponification  from  221.5  to 
232.4  milligrams  of  potassium  hydroxide,  while  an  equal  weight 
of  various  other  fats  and  oils  required  a  much  smaller  amount 
of  the  alkali.  The  following  table  gives  in  milligrams  the  quan- 
tity of  potassium  hydroxide  required  to  saponify  one  gram  of 
the  substances  named. 


Butter    .     . 

.  221.5  to  232.4 

Olive  oil 

.   189.3  to  192.6 

Lard  . 

.  192.0  to  196.5 

Palm  oil 

.  196.3  to  202.5 

Tallow    .     . 

.  193.2  to  198.0 

Palm-nut  oil 

.  220.0  to  247.6 

Butterine    . 

.  193.5  to  196.5 

Peanut  oil  . 

.  196.6 

Cocoanut  oil 

.  246.2  to  268.4 

Rape  oil 

.  170.2  to  176.4 

Cotton-seed  oil    191.0  to  196.6         Sesame  oil  .     .  190.0  to  192.4 

Experiments  made  in  this  laboratory  indicate  that  the  quan- 
tity of  alkali  required  to  saponify  one  gram  of  butter  obtained 
from  milk  by  evaporating  the  water  and  extracting  the  residue 
with  ether  is  quite  exactly  230  milligrams. 

It  was  found  by  Moore  that  one  gram  of  a  mixture  consisting 
of  49.3  per  cent  of  cocoanut  oil  and  50.7  per  cent  of  oleomar- 
garine required  for  its  saponification  220  milligrams  of  potassium 
hydroxide.  An  equal  quantity  of  another  mixture  of  the  same 
substances  containing  70.2  per  cent  of  cocoanut  oil  and  29.8 
per  cent  of  oleomargarine  required  234.9  milligrams. 


508  QUANTITATIVE  EXERCISES 

EXERCISE  LXII 

DETERMINATION   OF   THE  ALKALI  NEUTRALIZED   BY   THE 
SOLUBLE    AND    THE   INSOLUBLE  ACIDS 

METHOD  OF  MORSE  AND  BURTON 

There  are  required  for  this  determination  a  solution  of  potas- 
sium hydroxide  in  alcohol  and  one  of  hydrochloric  acid.  It  is 
not  necessary  to  know  the  strength  of  either  solution.  They 
should,  however,  be  quite  dilute  and  approximately  equivalent. 
The  solutions  used  in  the  preceding  exercise  will  answer  very 
well  for  this.  It  is  also  unnecessary  to  weigh  the  butter. 

Place  a  quantity  of  butter,  judged  to  weigh  not  more  than 
two  grams,  in  a  small  Erlenmeyer  flask  and  add  to  it  a  meas- 
ured but  excessive  quantity  of  the  alkali.  Attach  the  flask  to 
an  inverted  condenser  and  heat  it  on  a  water  bath  until  the 
saponification  is  complete.  Allow  the  flask  to  cool  to  the  tem- 
perature of  the  room  and  then  titrate  the  excess  of  the  alkali 
with  the  solution  of  hydrochloric  acid,  using  phenolphthalein  as 
the  indicator.  Attach  the  flask  to  a  condenser  arranged  for 
distillation.  Immerse  it  to  the  neck  in  water  and  distill  off  all 
the  alcohol,  collecting  the  distillate  for  subsequent  use.  Draw 
off  a  number  of  cubic  centimeters  of  the  alkali  equal  to  the 
number  which  was  added  to  the  butter  in  the  first  place  and 
titrate  with  the  acid.  Having  found  how  much  acid  is  required 
to  neutralize  the  whole  of  the  alkali,  and  also  the  alkali  which 
remained  after  saponification,  add  an  amount  of  the  acid  equal 
to  the  difference  between  these  quantities  to  the  soap  in  the 
flask.  It  is  also  well  to  add  from  50  to  75  cc.  of  water.  The 
quantity  of  acid  added  is,  of  course,  just  sufficient  to  liberate 
all  the  acids  of  the  butter.  Attach  the  flask  to  the  inverted 
condenser  and  heat  it  on  a  water  bath  until  the  insoluble  acids 
have  collected  in  an  oily  layer  at  the  top  and  the  liquid  below 
has  become  quite  clear.  The  minute  globules  of  molten  insol- 
uble acids  sometimes  remain  suspended  for  a  long  time,  giving 


BUTTER 


509 


the  liquid  a  milky  appearance.  If  filtration  is  attempted  while 
the  liquid  is  in  this  condition,  some  of  the  finely  divided  insol- 
uble acids  will  pass  through  the  filter.  The  difficulty  can  be 
remedied  in  either  of  two  ways :  first,  by  introducing  a  small 
quantity  of  long-fiber  asbestus  and  rotating  the  flask  for  a  few 
minutes ;  or,  second,  the  liquid  may  be  allowed  to  cool  and 
stand  for  a  few  hours.  In  the  latter  case  the  suspended  acid 
solidifies  and  rises  to  the  top,  and  on  reheating,  the  liquid  below 
the  oily  layer  is  usually  found  to  be  free  from  suspended  glob- 
ules. Filter  through  a  double-thick  paper  which  has  been  satu- 
rated with  water,  and  wash  the  insoluble  acids  as  directed  under 
LIX.  Add  the  alcoholic  distillate  to  the  filtrate  and  titrate 
with  the  alkali.  Dissolve  the  insoluble  acids  in  hot  alcohol 
and  titrate  them  with  some  of  the  same  alkali. 

Express  the  relation  of  the  quantities  of  the  alkali  neces- 
sary to  neutralize  the  soluble  and  the  insoluble  acids  in  the  form 
of  percentages  of  the  whole  amount  of  the  alkali  which  was 
required  to  saponify  the  butter. 

It  is  evident  that  the  sum  of  the  volumes  of  the  alkali  used 
in  neutralizing  the  two  classes  of  acids  must  equal  the  volume 
of  the  alkali  required  for  the  saponification.  This  fact  fur- 
nishes a  check  of  some  value  upon  the  correctness  of  the  work. 

The  following  table  gives  the  percentages  of  alkali  required 
to  neutralize  the  soluble  and  the  insoluble  acids  in  butter,  cocoa- 
nut  oil,  cotton-seed  oil,  oleomargarine,  lard,  and  tallow. 


Per  cent  KOH 
required  for 
Insoluble  Acids 

Per  cent  KOH 
required  for  , 
Soluble  Acids 

Butter    

86  57 

13  17 

Cocoanut  oil  (unwashed)   .              .... 

91  95 

8  17 

Cocoanut  oil  (washed  with  hot  water)      .     . 
Cocoanut  oil  (washed  with  dilute  Na2C03)    . 
Cotton-seed  oil     

92.43 
92.33 
92  05 

7.42 
7.45 

7  76 

Oleomargarine      

95  40 

4.57 

Lard       .               .          .... 

95  96 

3  82 

Beef  tallow      

96  72 

3  40 

510  QUANTITATIVE  EXERCISES 

It  was  found  by  Moore  that  a  mixture  of  50,  per  cent  of  but- 
ter, 27.5  per  cent  of  oleomargarine,  and  22.5  per  cent  of  cocoanut 
oil  could  not  be  distinguished  from  genuine  butter,  either  by  the 
method  of  Hehner  or  by  that  of  Koettstorfer.  Such  a  mixture, 
examined  by  this  method,  gave  90.17  per  cent  KOH  required  for 
insoluble  acids;  9.70  per  cent  KOH  required  for  soluble  acids. 

EXERCISE  LXIII 

DETERMINATION  OF   THE  IODINE  ABSORBED 
HUBL'S  METHOD 

Dissolve  iodine,  12.5  grams,  and  mercuric  chloride,  15  grams, 
each  in  250  cc.  of  95  per  cent  alcohol.  Filter  the  mercuric  chlo- 
ride solution,  if  necessary.  Mix  the  two  solutions  and  allow 
the  mixture  to  stand  for  twelve  hours. 

Dissolve  about  24  grams  of  sodium  thiosulphate  in  water  and 
dilute  to  a  liter.  Determine  the  strength  of  the  solution  in  the 
usual  manner  with  resublimed  iodine. 

Test  the  chloroform  which  is  to  be  used  by  adding  to  10  cc.  of 
it  10  cc.  of  the  iodine  solution.  After  three  hours  determine  the 
iodine  in  the  chloroform  and  also  in  10  cc.  of  the  original  iodine 
solution.  The  quantities  of  iodine  found  should  be  the  same. 

Dissolve  25  grams  of  potassium  iodide  in  250  cc.  of  water. 

Prepare  a  one  per  cent  starch  solution  in  the  usual  manner. 

Weigh  about  0.8  gram  of  butter  into  a  200-cc.  Erlenmeyer 
flask.  Dissolve  it  in  chloroform.  Add  20  cc.  of  thev iodine  solu- 
tion. Close  the  flask  with  a  rubber  stopper  and  shake  it.  If  the 
solution  becomes  cloudy,  add  a  little  more  chloroform.  Close 
the  flask  and  allow  it  to  stand  2  hours.  If  the  color  of  the 
solution  fades  to  any  considerable  extent,  add  more  of  the  iodine 
solution.  The  excess  of  the  iodine  must  be  quite  large.  Add 
from  10  to  15  cc.  of  the  potassium  iodide  solution  and  shake  well. 
Dilute  with  150  cc.  of  water.  Titrate  with  the  thiosulphate  solu- 
tion until  the  color  becomes  quite  faint,  then  add  starch  paste  and 


BUTTER 


511 


titrate  to  the  end-reaction.  Draw  off  a  quantity  of  the  iodine 
solution  equal  to  that  added  to  the  butter  in  the  first  place.  Add 
potassium  iodide  and  determine  the  iodine  in  it.  The  difference 
between  the  quantities  of  iodine  found  by  the  two  titrations  is 
equal  to  the  quantity  which  has  been  absorbed  by  the  butter. 

The  quantity  of  the  iodine  absorbed  is  stated  in  the  form  of 
percentage,  by  weight,  of  the  butter  taken.  The  number  of 
parts  of  iodine  absorbed  by  one  hundred  parts  of  any  fat  or  oil  is 
known  as  its  "  iodine  number."  The  following  table  gives  the 
iodine  number  of  several  of  the  fats  and  oils. 


HUBL 

MOOEE 

ARCHBUTT 

97.5-98.9 
81.6-84.5 
97.0-105.0 
105.0-108.0 
105.0-108.0 
156.0-160.0 
148.0 
84.0-84.7 

98.1 
83.0 
101.0-104.0 
102.7 
106.0-109.0 
155.2 

Olivp  nil 

Linseed  oil  (raw)  
Linseed  oil  (boiled)    .... 
Castor  oil                          .     . 

84.3 
147.9 
84.3 
71.9 

Sperm  oil     .                .... 



Neat's-foot  oil 

66.0 
50.4-52.4 
8.9 
34.0 
4.2 
26.0-35.1 
55.3 
59.0 
40.0 

Palm  oil 

50.3 
8.9 

Japan  wax 

Butter 

19.5-38.0 
50.0 
61.9 

Oleomargarine            .... 

Lard 

Tallow 

It  will  be  observed  that  the  iodine  number  of  butter,  as  given 
in  the  table,  varies  between  wide  limits.  The  lower  values, 
however,  were  obtained  from  samples  which  were  in  an  advanced 
stage  of  decomposition. 

It  has  been  shown  by  Moore  that  the  addition  of  cocoanut  oil  in 
certain  proportions  to  lard,  to  oleomargarine,  and  to  a  mixture  of 


512  QUANTITATIVE  EXERCISES 

butter  and  oleomargarine,  gives  products  whose  iodine  numbers 
are  about  equal  to  that  of  genuine  butter.  It  was  also  found  by 
him,  as  previously  stated,  that  certain  mixtures  of  cocoanut  oil 
with  other  fats  yield  the  same  results  as  butter  when  tested  by 
the  methods  of  Hehner  and  Koettstorfer.  All  such  mixtures 
are,  however,  readily  distinguished  from  butter  by  the  method 
of  Reichert. 

It  is  doubtful  whether  cocoanut  oil  —  which,  if  it  were  added 
in  the  right  proportions,  would  render  the  methods  of  Hehner, 
Koettstorfer,  and  Hubl  useless  in  butter  analysis  —  is,  or  can  be, 
successfully  employed  in  adulterating  butter  or  in  the  manu- 
facture of  its  substitutes.  Its  taste  and  odor  seem  to  render 
such  a  use  of  it  impracticable. 

The  chief  value  of  Hubl's  method  lies  in  its  efficiency  as  a 
means  of  detecting  the  adulterations  of  oils. 

EXERCISE  LXIV 

DETERMINATION  OF  BUTTER  IN  MILK 
I.   DETERMINATION  BY  THE  ORDINARY  GRAVIMETRIC  METHOD 

To  about  one  liter  of  commercial  ether,  in  a  strong  bottle, 
add  thin  shavings  of  metallic  sodium.  Close  the  bottle  with  a 
cork  having  a  groove  on  one  side,  or  with  one  through  which 
a  glass  tube  with  a  small  onitlet  has  been  passed.  When  the 
sodium  becomes  so  much  incrusted  as  to  prevent  contact 
between  the  ether  and  the  metal,  clean  it  by  pressing  and  rub- 
bing the  pieces  against  the  bottom  of  the  bottle  with  a  stout 
glass  rod.  Continue  to  clean  the  metal  from  time  to  time,  add- 
ing more  of  it,  if  necessary,  until  the  evolution  of  hydrogen 
ceases.  Keep  the  ether  in  the  bottle  with  an  excess  of  sodium 
until  it  is  needed  for  the  extraction  of  the  butter. 

Ignite  some  asbestus  in  a  platinum  dish  over  a  blast  lamp  and 
place  a  portion  of  it  in  a  "  Hofmeister  "  capsule,  —  a  thin  glass 
shell  which  can  be  readily  crushed. 


BUTTER  513 

Measure  10  cc.  of  well-shaken  milk  into  a  large  weighing 
glass.  Close  the  glass  and  weigh  it.  Pour  the  milk  upon  the 
asbestus  in  the  capsule.  Close  the  glass  and  weigh  it  again. 

Evaporate  to  dryness  in  a  steam  bath  consisting  of  a  liter- 
Erlenmeyer  flask,  and  a  funnel  having  a  diameter  of  about 
120  mm.  at  the  top  and  a  wide  neck.  Pass  the  neck  <=* 
of  the  funnel  through  a  cork  which  fits  the  flask,  and  J 
provide  a  rest  for  the  capsule  by  placing  a  small  tri- 
angle in  the  bottom  of  the  funnel,  or  by  hanging  glass 
rods  with  curved  ends  over  the  edge  of  it. 

Prepare  a  filter  for  the  Soxlet  apparatus, 
Figs.  71a  and  71b,  which  is  to  be  used  in  the 
extraction.  For  this  purpose  a  cylinder  of 
wood  or  other  material  is  required.  It  must 
be  square  and  smooth  at  one  end,  and  of  some- 
what smaller  diameter  than  the  extraction 
apparatus.  Wind  a  strip  of  the  best  and 
strongest  filter  paper  twice  around  the  cylinder,  allow- 
ing one  edge  to  extend  considerably  beyond  the  square 
end,  and  fasten  it  with  pins.  Fold  in  the  free  edge 
upon  the  end  of  the  cylinder  and  press  the  folds  against 
some  hard  object.  If  necessary,  the  folds  may  be 
secured  in  their  places  by  wiring  them  together.  If  it 
is  feared  that  the  bottom  of  the  filter  will  permit  solid 
material  to  pass  through  it,  a  little  paper  pulp  sus- 
pended in  water  may  be  poured  into  the  filter  and 
the  water  drawn  off  with  the  pump  as  in  the  prepara- 
tion of  a  Gooch  filter.  The  filter  should  be  long 
enough  to  extend  from  the  bottom  of  the  extractor 
to  the  point  where  the  ether  vapors  enter  it,  and  so 
wide  that  no  place  will  be  left  between  the  paper 
and  the  glass. 

Filters  especially  made  for  the  Soxlet  apparatus  can  now 
be  obtained  and  are  to  be  preferred  to  the  one  described 
above. 


514  QUANTITATIVE  EXERCISES 

Distill  on  the  water  bath  a  quantity  of  the  ether  which  will 
somewhat  more  than  fill  the  extractor.  Place  the  dry  filter  and 
a  roll  of  filter  paper  in  the  extractor  and  extract  them  with  the 
redistilled  ether  for  2  hours.  The  heat  should  be  so  regu- 
lated during  the  extraction  that  the  apparatus  will  fill  and 
siphon  off  in  four  or  five  minute  periods.  Great  care  must  be 
exercised  in  selecting  the  corks  by  which  the  different  parts  of 
the  apparatus  —  the  inverted  condenser,  the  extractor,  and  the 
flask  —  are  joined  together.  The  flask  should  be  heated  in  an 
air  bath  and  not  in  water.  A  thin  iron  crucible  somewhat  lar- 
ger than  the  flask  answers  very  well  as  a  bath.  The  heat  rising 
from  the  bath  sometimes  interferes  with  the  automatic  action  of 
the  siphon.  This  difficulty  is  easily  remedied  by  placing  a 
shield  of  cardboard  between  the  bath  and  the  extractor. 

Place  the  capsule  containing  the  dried  milk,  bottom  upwards, 
in  a  porcelain  mortar.  Cover  the  mortar  with  a  piece  of  card- 
board having  a  hole  in  the  center  just  large  enough  for  the 
pestle.  Hold  it  in  place  with  the  fingers  of  one  hand,  and, 
with  the  pestle  in  the  other  hand,  crush  and  grind  the  capsule 
and  its  contents.  Transfer  the  ground  material  to  the  filter, 
and  place  it  in  the  extractor.  Pour  a  little  ether  into  the  mor- 
tar and  rub  it  over  the  inside  with  the  pestle.  Then  pour  the 
ether  down  a  glass  rod  into  the  filter.  Continue  washing  the 
mortar  in  the  same  mariner  with  small  quantities  of  ether  until 
it  is  believed  that  all  the  fat  has  been  removed  from  it.  Cover 
the  contents  of  the  filter  with  some  of  the  extracted  paper. 
Attach  a  weighed  flask  containing  ether  to  the  extractor,  and 
extract  the  filter  for  not  less  than  4  hours.  Distill  off  the 
ether  and  dry  the  butter  residue  to  a  constant  weight  at  a  tem- 
perature of  60°  or  70°.  During  the  latter  part  of  this  process 
a  current  of  dried  air  should  be  drawn  through  the  flask  to 
facilitate  the  removal  of  the  last  traces  of  the  ether. 

Having  found  the  weight  of  the  butter,  determine,  by  the 
method  of  Koettstorfer,  how  much  potassium  hydroxide  is 
required  to  saponify  it. 


BUTTER  515 

The  ether,  when  not  in  use,  should  be  returned  to  the  main 
supply  in  the  bottle  containing  metallic  sodium. 

II.    DETERMINATION  BY  ADAMS'  METHOD 

Cut  off  strips  of  thick  filter  paper,  65  mm.  wide  and  600  mm. 
long.  Roll  them  up  loosely  and  extract  thoroughly  with  anhy- 
drous ether.  Measure  5  cc.  of  well-shaken  milk  into  a  weighing 
glass  and  weigh.  Open  one  of  the  rolls  and  stretch  the  strip  of 
paper  —  horizontally  and  at  some  distance  above  the  table  —  be- 
tween two  clamps.  Distribute  the  milk  as  uniformly  as  possible 
over  the  central  portion  of  the  paper.  Close  the  glass  from 
which  the  milk  was  taken  and  weigh  it.  Place  a  heated  air  bath 
under  the  paper,  and  when  it  appears  to  be  nearly  dry,  roll  it 
up  loosely.  Place  the  roll  on  a  watch  glass  in  the  air  bath  and 
heat  for  an  hour  at  100°.  Put  the  roll  into  the  filter  in  the 
extractor  and  extract  with  anhydrous  ether  for  not  less  than 
3  hours.  Proceed  with  the  extract  as  directed  under  I. 

Instead  of  opening  the  roll  of  extracted  paper  and  distribut- 
ing the  milk  in  the  manner  described  above,  the  roll  may  be 
tied  with  a  thread  and  one  end  of  it  dipped  into  the  milk. 
When  the  milk  has  all  been  absorbed,  the  roll  is  placed  on  the 
unsoiled  end  in  a  watch  glass  and  dried  at  100°. 

Having  found  the  weight  of  the  butter,  determine  how  much 
alkali  is  required  to  saponify  it. 


III.   DETERMINATION  BY  THE  METHOD  OF  MORSE,  PIGGOT,  AND 

BURTON 

In  this  method  the  milk  is  dried  by  means  of  anhydrous  cop- 
per sulphate  and  the  fat  extracted  either  with  the  low-boiling 
products  of  petroleum  or  with  ether.  In  the  former  case  the 
quantity  of  the  butter  is  to  be  determined  by  saponification. 

Prepare  a  quantity  of  anhydrous  copper  sulphate  by  heating 
the  recrystallized  salt  in  a  porcelain  dish  at  a  temperature  of 


516  QUANTITATIVE   EXERCISES 

250°.  It  may  be  prepared  more  expeditiously  by  heating  the 
salt  upon  a  copper  plate  with  the  naked  flame;  but,  in  that 
case,  the  material  must  be  constantly  stirred  and  not  allowed 
to  harden  upon  the  metal. 

In  a  porcelain  mortar  —  provided  with  a  lip  and  having  a 
diameter  of  90  or  100  mm.  at  the  top  —  place  about  20  grams 
of  the  anhydrous  copper  sulphate.  Make  a  depression  in  the 
center  with  the  pestle,  and  pour  into  this  a  weighed  10-cc. 
portion  of  milk.  Draw  the  copper  sulphate  which  was  pushed 
aside  in  making  the  cavity  into  and  over  the  milk,  and,  while 
the  mass  is  still  somewhat  moist,  begin  to  grind  with  the  pestle. 
Continue  the  grinding  until  the  material  is  reduced  to  a  fine 
powder,  and  extract  it  with  ether  in  the  Soxlet  apparatus. 


CHAPTER  XXIII 
ELECTRIC  HEATING  APPLIANCES  FOR  LABORATORY  USE 

The  advantages  of  employing  electricity  for  heating  purposes 
in  the  laboratory  are  numerous. 

1.  The  heat  required  can  be  generated  within  the  space  to  be 
heated,  —  that  is,  where  it  is  needed. 

2.  Since  the  heat  is  generated  where  needed,  it  can  be  econ- 
omized very  effectively  by  surrounding  the  heated  space  with 
nonconducting  materials  and  confined  air  spaces. 

3.  With  practically  constant  electromotive  force,  such  as  we 
have  in  the  storage  battery,  the  temperatures  can  be  controlled 
with  a  high  degree  of  exactitude. 

4.  Having  once  calibrated  an  electric  heating  device,  —  that 
is,  having  determined  for  each  temperature  the  resistance  of  the 
portion  of  the  circuit  in  which  the  heat  is  generated,  —  one 
can  thereafter  at  any  time  ascertain  the  temperature  without 
the  aid  of  thermometer  or  pyrometer. 

5.  Where  calibrated  electric  furnaces  or  baths  are  used,  it  is 
feasible  in  many  cases  to  ascertain  at  what  temperatures  reactions 
take  place  and  to  determine  when  they  begin  and  cease. 

6.  There  is  no  contact  of  the  substances  heated,  with  dele- 
terious products  of  combustion,  as  often  happens  when  gas  is 
used. 

7.  Heating  by  electricity  is  much  less  destructive  than  heat- 
ing by  gas  to  the  materials  used  in  the  construction  of  furnaces, 
baths,  etc. 

8.  Owing  to  the  less  destructive  effects  upon  costly  materials 
and  to  the  practicability  of  preventing  waste  through  loss  of 
heat  to  the  outside  air,  and  of  so  adjusting  the  resistance  of  the 
heating  device  to  the  electromotive  force  of  the  current  that 

517 


518 


QUANTITATIVE  EXERCISES 


little  is  lost   in    external  resistance,  heating  by  electricity  is, 
in  many  operations,  more  economical  than  heating  by  gas. 

Some  of  the  appliances  in  use  in  the  author's  laboratory  in 
which  the  electric  current  is  utilized  for  heating  purposes  are 
here  described. 

1.  CRUCIBLE  FURNACE 

Figure  72  represents  a  furnace  for  heating  materials,  —  fusion 
of  silicates,  etc.,  in  platinum  crucibles.  The  three  hard-burned 
perforated  clay  rings,  a,  5,  and  c,  are  held  in  place  by  three 

platinum  rods  made  from 
No.  16  wire  (B.  &  S.  gauge). 
The  ends  of  these  rods  are 
threaded  and  each  end  is  pro- 
vided with  two  small  plati- 
num nuts  between  which  the 
rings  a  and  c  are  firmly  fas- 
tened. The  purpose  of  the 
ring  b  is  to  keep  the  crucible 
from  coming  in  contact  with 
the  platinum  wires.  It  is 
adjustable  up  or  down,  but 
may  be  fixed  in  any  desired 
position  by  twisting  short 
pieces  of  wire  about  the  rods. 
The  internal  diameters  of  b 
and  c  are  equal,  while  that  of 
a  is  smaller  in  order  to  fur- 
nish a  rest  for  the  crucible  support.  Each  ring  is  provided 
with  60  perforations  in  two  rows  of  30  each,  through  which 
the  platinum  wire  (No.  26  B.  &  S.  gauge)  is  threaded  up  and 
down  in  the  manner  shown  in  the  figure.  The  inner  row  of 
wires  also  crosses  from  side  to  side  under  a.  The  total  height 
of  the  furnace,  as  represented  in  the  figure,  is  80  mm.,  and  the 
total  length  of  the  wire  is  16  feet.  The  platinum  rods  serve 


FIG.  72 


LABORATORY  ELECTRIC  HEATING  APPLIANCES      519 


FIG.  73 


two  important  purposes.     They  support  the  whole  weight  of 

the  furnace  and  its  contents,  and  thus  prevent  any  stretching 

of  the  wires  while  hot ;  and,  having  at  all 

times  about  the  same  temperature  as  the 

wires,  they  keep  the  latter  perfectly  straight, 

and  thus  prevent  any  danger  of  short  circuits 

through  contact  between  adjacent  strands. 
The  furnace  as  represented  in  Fig.  72  is 

surrounded  by  the  clay  cylinder  c?,  as  shown 

in  Fig.  73,  the  ring  c,  Fig.  72,  resting  upon 

the   upper  edge  of  d.     The  cylinder  d  is 

provided  with  the  perforated  cover  e  and  the 

bottom  /  which 
li  rests  upon  the 
three  truncated 
clay  cones  g. 
The  hole  in  e 
serves  for  the 

introduction  of  a  thermometer  or 
pyrometer.  Figure  74  represents  a 
cylinder  h  of  larger  dimensions 
which  surrounds  d.  It  is  also  fur- 
nished with  a  perforated  cover  i,  a 
bottom  j,  and  the  supports  k. 

The  complete  furnace  is  shown  in 
Fig.  75,  in  which  I  is  a  Le  Chatelier 
pyrometer  ;  m,  a  galvanized  iron  cyl- 
inder with  cover,  —  both  of  which 
are  lined  and  covered  with  asbestus 
paper  ;  w,  a  series  of  asbestus  boards 
with  ventilating  spaces  between 
them  ;  o,  a  block  of  soapstone  ;  and  p, 
a  wooden  board  on  which  the  whole 

arrangement  rests  and  can  be  moved  without  disturbing  any  of 

the  parts  ;  q  and  r  are  firmly  fixed  steel  rods  which  serve  to 


FIG.  74 


520  QUANTITATIVE  EXERCISES 

connect  the  wires  coming  from  the  furnace  with  the  external 
circuit  and  to  protect  the  former  from  injury  through  strains  of 
any  kind ;  *,  *  are  short  pieces  of  pipestem  which  keep  the  plat- 
inum wires  from  contact  with  the  metallic  cylinder  m.  The 
crucible  support  t  is  made  as  light  and  open  as  possible,  coming 
in  contact  with  the  crucible  at  three  points  only. 

The  method  of  making  the  clay  parts  and  the  process  of  wir- 
ing this  furnace  have  been  described  in  the  American  Chemical 
Journal  for  1904. 

The  first  step  to  be  taken  with  a  furnace  of  this  kind  is  to 
calibrate  it,  —  that  is,  to  determine  its  resistance  for  every  tem- 
perature up  to  the  limit  to  which  it  is  afterwards  to  be  heated ; 
for  it  will  then  be  feasible  at  all  times,  without  the  aid  of  a 
thermometer  or  pyrometer,  to  maintain  in  it  with  certainty  any 
desired  temperature,  or  to  ascertain  at  any  time  what  the  tem- 
perature in  it  is.  The  furnace,  in  fact,  becomes,  by  virtue  of 
such  a  calibration,  a  resistance  pyrometer. 

For  the  calibration  of  the  furnace  in  use  in  the  author's  labo- 
ratory there  were  employed  a  mercury  thermometer  registering 
to  550°,  a  Le  Chatelier  platinum  platinum-rhodium  pyrometer, 
and  a  Keiser-Schmidt  decimillivoltmeter,  all  of  which  had  been 
tested  at  the  German  Physikalisch-Technischen  Reichsanstalt. 
We  give  below  in  tabular  form  the  data  for  the  furnace  up  to 
967°.  The  calibration  was  not  carried  higher  because  the  tem- 
perature 950°  is  sufficient  for  ordinary  crucible  work.  The 
temperatures  given  are  probably  correct  to  within  5°  or  less. 
The  voltmeter  which  was  used  to  measure  the  electromotive 
force  which  was  developed  at  the  junction  of  the  platinum  and 
platinum-rhodium  wires  is  furnished  with  two  scales,  —  one  in 
which  each  division  has  the  value  of  a  decimillivolt,  and  another 
in  which  a  division  corresponds  to  20°  of  temperature.  Not- 
withstanding the  coarseness  of  the  needle  in  this  instrument, 
the  errors  of  parallax  in  reading  could  not  have  amounted  to 
more  than  one-fourth  of  a  division  on  either  scale. 


LABORATORY  ELECTRIC   HEATING  APPLIANCES     521 


522 


QUANTITATIVE  EXERCISES 


CURRENT 

VOLTAGE 

RESISTANCE 

WATTS 

TEMPERATURE 

2.5 

33.7 

13.48 

84.25 

383° 

2.6 

36.0 

13.846 

93.60 

407 

2.7 

38.5 

14.259 

106.38 

434 

2.8 

41.0 

14.671 

114.80 

463 

2.9 

43.6 

15.034 

126.44 

488 

3.0 

47.4 

15.80 

142.20 

526 

3.1 

50.2 

16.193 

155.62 

558 

3.2 

52.7 

16.468 

168.72 

587 

3.3 

55.3 

16.757 

182.49 

615 

3.4 

58.3 

17.147 

198.22 

640 

3.5 

61.2 

17.486 

214.20 

663 

3.6   ' 

64.3 

17.861 

231.48 

690 

3.7 

66.9 

18.081 

247.53 

710 

3.8 

70.1 

18.447 

266.38 

741 

3.9 

73.3 

18.795 

285.87 

766 

4.0 

76.3 

19.075 

305.20 

793 

4.1 

79.5 

19.390 

325.55 

813 

4.2 

82.5 

19.643 

338.25 

840 

4.3 

85.3 

19.837 

366.79 

867 

4.4 

88.7 

20.159 

390.28 

888 

4.5 

92.0 

20.444 

414.00 

914 

4.6 

95.0 

20.652 

437.00 

940 

4.7 

98.0 

20.851 

460.60 

967 

Unless  absolutely  unavoidable,  a  calibrated  furnace  like  the 
foregoing  should  not  be  heated  above  1150°.  At  higher  tem- 
peratures the  metal  is  volatilized  to  an  extent  which  sensibly 
affects  the  resistance  of  the  wires,  making  a  recalibration 
necessary. 

It  will  be  readily  understood  that  an  accurately  calibrated 
resistance  furnace  is  theoretically  an  admirable  instrument  for 
the  study  of  reactions  at  high  temperatures;  for,  if  external 
conditions  are  constant,  it  will  always  require  the  same  amount 
of  electrical  energy  to  maintain  any  given  temperature  in  the 
empty  furnace,  or,  what  is  the  same  thing,  a  given  amount  of 
electrical  energy  is  bound  to  maintain  in  it  always  a  certain  fixed 


LABORATORY  ELECTRIC  HEATING  APPLIANCES      523 

temperature.  If  now  any  reaction  involving  the  disappearance 
or  evolution  of  heat  takes  place  within  the  heated  space,  the 
amount  of  electrical  energy  required  to  maintain  the  tempera- 
ture at  which  it  takes  place  will  be  increased  or  diminished 
according  as  the  reaction  is  endothermic  or  exothermic.  And 
the  difference,  if  it  can  be  ascertained,  will  serve  as  a  measure 
of  the  heat  energy  of  the  reaction.  There  is  little  difficulty  in 
discovering  at  what  temperature  reactions  involving  consider- 
able heat  effects  take  place  in  such  an  electric  furnace.  If,  for 
instance,  a  substance  is  losing  water  in  the  furnace,  it  will  be 
noticed  that  the  amount  of  current  required  to  maintain  the 
temperature  as  determined  by  the  resistance  is  considerably 
greater  than  that  required  to  maintain  the  same  temperature  in 
the  empty  furnace,  showing  that  a  reaction  involving  the  loss 
of  heat  is  going  on  within.  Again,  when  a  fusible  substance  is 
heated  in  the  furnace  and  the  temperature  of  fusion  is  reached, 
there  is  noticed  an  increase  in  the  flow  of  the  current  without 
any  corresponding  increase  in  the  resistance  of  the  platinum 
wire,  i.e.  in  the  temperature  within  the  furnace.  The  increased 
flow  of  current  continues  for  a  time  and  then  diminishes  to  its 
original  volume,  showing  that  the  fusion  is  finished.  If  the 
furnace  has  been  calibrated,  a  determination  of  its  resistance 
during  the  time  of  the  increased  flow  of  current  will  give  the 
melting  point  of  the  substance.  In  this  way  it  was  found  that 
the  fusing  point  of  sodium  carbonate  is  between  860°  and  863°. 
The  exact  time  of  fusion  may  also  be  ascertained  by  arranging 
to  have  the  sinking  of  a  platinum  weight  into  the  softened 
material  either  close  or  break  a  circuit  in  which  there  is  a 
sensitive  ammeter. 

Notwithstanding  the  favorable  location  of  the  wires  in  this 
furnace  with  respect  to  the  space  heated,  and  the  double- 
inclosed  air  spaces  surrounding  it,  the  temperature  within  does 
not  immediately  rise  to  its  maximum  when  the  current  passing 
through  the  furnace  is  increased,  —  that  is,  some  time  elapses 
before  the  temperature  within  adjusts  itself  to  that  of  the 


524  QUANTITATIVE  EXERCISES 

external  atmosphere.  The  tardiness  of  this  adjustment  between 
internal  and  external  conditions  lessens  somewhat  the  precision 
of  the  results  obtained  when  the  furnace  is  employed  for  the 
investigation  of  reactions  at  high  temperatures.  It  is,  however, 
to  be  hoped  that  a  remedy  will  be  found  for  this  defect  of  a 
method  which  is  otherwise  so  promising. 

2.  AIR  BATHS  WITH  GRAPHITE  STOVES 

Graphite  is  a  substance  which  conducts  electricity,  but  differs 
from  metallic  conductors  in  that  its  conductivity  increases  with 
rise  of  temperature.  If  it  could  be  evenly  distributed  in  suffi- 
cient quantity  over  a  smooth  surface  and  made  to  adhere  to  the 
same  with  sufficient  tenacity,  it  should  afford  a  ready  means  of 
raising  temperature  by  electricity  up  to  that  at  which  graphite 
is  burned  by  the  oxygen  of  the  air. 

Unfortunately  the  rubbing  of  ground  dry  graphite  upon  a 
smooth  surface  does  not  give  a  sufficiently  heavy  coating  of  the 
material  to  conduct  the  requisite  amount  of  current.  A  thicker 
covering  can  be  obtained  by  repeated  applications  of  it,  in  a 
wet  condition,  with  a  flat  camel's-hair  brush.  The  surface  to 
which  it  is  applied  must,  however,  be  hot  enough  to  evaporate 
the  water  almost  instantaneously ;  otherwise  the  deposit  will  be 
quite  uneven.  The  results  obtained  by  the  last  method  are 
nevertheless  unsatisfactory.  The  material  lacks  adhesiveness, 
and  when  an  attempt  is  made  to  harden  it  by  polishing  with  a 
stiff  brush,  much  of  the  graphite  is  detached,  and  the  surface 
which  is  produced  is  easily  injured  by  handling  or  by  contact 
with  other  objects. 

To  secure  a  satisfactory  deposit,  some  other  material  must  be 
mixed  with  the  graphite,  and  all  of  the  substances  which  lend 
themselves  to  this  purpose  are  incapable  of  withstanding  as  high 
temperatures  as  graphite  itself.  There  are  two  substances  with 
which  the  graphite  may  be  mixed  with  excellent  results  so  far 
as  the  conducting  surface  is  concerned:  first,  washed  and  bolted 


LABORATORY  ELECTRIC   HEATING  APPLIANCES      525 


FIG.  76 


clay,  and  second,  some  kinds  of  soap.     Of  these  the  former  is 
much  to  be  preferred. 

The  best  results  are  secured  with  the  paste  form  of  a  well- 
known  stove  polish  which  is  obtainable  everywhere  in  this  coun- 
try, and  this  material  is  to  be  recommended  for  the  preparation 
of  graphite  stoves 
above  any  mix- 
ture of  graphite 
with  clay  or  with 
soap  which  can 
be  made  in  the 
laboratory. 

Soapstone  is 
undoubtedly  the 
best  of  all  available  materials  on  which  to  spread  the  graphite 
for  heating  purposes.  It  is  easily  cut  to  any  desired  form ;  its 
surface  can  be  made  very  smooth,  which  is  essential  to  the  best 
results  ;  and,  above  all,  it  withstands  better  than  any  other  non- 
conducting material  great  and  sudden  changes  of  temperature. 
Figures  76  and  77  represent  two  forms  of  graphite  stoves  for 
ordinary  hot-air  baths,  a,  Fig.  76,  is  a  soapstone  block  of  con- 
venient size  for 
the  usual  rec- 
tangular copper 
air  bath.  It  is 
seen  in  place  in 
Fig.  78.  6,  5,  J, 
b  are  strips  of 
iron  which  are 
bolted  to  the 
block  at  the 
ends,  the  central  bolt  at  each  end  serving  for  the  attachment  of 
the  wires  of  the  circuit.  The  corner  bolts,  which  are  longer 
than  the  others,  are  covered  with  the  soapstone  caps  <?,  c,  c,  c. 
On  these  rests  the  perforated  metallic  shelf,  as  seen  in  Fig.  78. 


FIG.  77 


526 


QUANTITATIVE  EXERCISES 


Figure  77  represents  another  arrangement  for  heating  the  same 
bath,  which  has  the  advantage  that  the  surfaces  on  which  the 
heat  is  generated  are  vertical,  a,  a,  a,  a  are  strips  of  soapstone 
of  such  dimensions  that  when  bolted  together  by  twos,  as  shown 
in  the  figure,  three  pairs  of  them  can  rest  on  the  bottom  of  the 
bath,  leaving  room  between  the  ends  for 
the  insertion  of  mica  plates  to  prevent 
electrical  contact.  Each  end  of  every 
soapstone  strip  is  provided  with  two  iron 
plates  which  are  bolted  to  the  stone  in 
the  manner  shown.  Two  strips,  thus 
furnished,  are  fastened  together  by  the 
longer  bolts  6,  5,  which  are  also  utilized 


FIG.  78 

for  the  attachment  of  wires.  Figure  78  represents  the  arrange- 
ment as  a  whole,  b  is  a  copper  bath  of  the  usual  form  having  a 
mica  window  in  the  door.  The  stove  «,  which  may  be  of  either 
of  the  forms  represented  in  Figs.  76  and  77,  rests  on  the  soap- 
stone  strips  <?,  c.  The  inner  vertical  walls  of  the  copper  bath 
may  be  lined  to  advantage  with  thin  asbestus  board,  but  in 


LABORATORY    KLKCTRIC    1 1  HATING  APPLIANCES      527 

that  case  it  is  well  to  cover  the  exposed  surface  of  the  board 
with  aluminium  paint,  which  will  prevent  the  lodgment  of  dust 
and  the  shedding  of  particles  of  asbestus.  The  wires  of  the 
stove  pass  horizontally  through  the  sides  of  the  copper  bath  and 
the  outer  compartment  to  the  exterior  of  the  wooden  box  which 
incloses  the  whole,  where  each  wire  terminates  in  a  binding 
post  screwed  into  the  box.  Where  the  wires  pass  through  the 
copper  walls  they  are  prevented  from  coming  in  contact  with 
the  metal  by  short  pieces  of  clay  pipestem.  When  a  stove 
such  as  is  shown  in  Fig.  77  is  used,  each  of  the  three  pairs  of 
plates  is  provided  with  independent  wires  and  binding  posts, 
any  required  combination  of  the  individual  pairs,  in  parallel  or 
in  series,  being  made  more  conveniently  on  the  outside  than  on 
the  inside  of  the  bath,  d  is  a  diminutive  electric  lamp  of  small 
voltage,  which  serves  to  illuminate  the  scales  of  the  ther- 
mometers. The  current  for  it  comes  through  a  shunt  from  the 
stove,  and  a  switch  is  fastened  to  the  outside  of  the  box  for  the 
purpose  of  closing  and  breaking  the  lamp  circuit.  The  voltage 
of  the  lamp  must  be  suited,  of  course,  to  the  fall  in  potential  of 
the  stove.  The  interior  of  the  wooden  box  is  lined  on  all  sides 
except  the  front  with  asbestus  board  so  as  feo  leave  confined  air 
spaces  between  the  asbestus  and  the  wood.  The  surface  of  the 
lining  boards  is  also  covered  with  aluminium  paint.  /  is  a 
closely  fitting  shelf  of  asbestus  board  (also  painted)  which  rests 
upon  the  copper  bath  and  is  supported  at  the  ends,  g  is  a  door 
which  slides  in  grooves  in  the  wooden  box.  It  contains  two 
mica  windows  and  is  made  from  asbestus  board. 

The  shelf/ divides  the  interior  of  the  box  into  two  compart- 
ments whose  temperatures  will  differ  quite  widely  from  each 
other  and  from  the  temperature  which  is  kept  in  the  copper 
bath.  This  difference  can,  however,  be  diminished,  if  desired, 
by  replacing  /  by  a  more  or  less  open  shelf  which  permits  circu- 
lation of  air  between  the  upper  and  lower  spaces.  Each  of  the 
heated  compartments,  including  the  copper  bath,  should  be  pro- 
vided with  two  properly  located  ventilating  tubes,  running  to 


528 


QUANTITATIVE  EXERCISES 


the  outside  of  the  wooden  box.  Porcelain  insulating  tubes  are 
well  adapted  to  this  purpose.  They  may  be  closed  with  corks 
to  prevent  loss  of  heat  when  not  needed. 

The  data  which  were  obtained  in  testing  two  such  baths  as 
have  been  described  are  given  below.  They  will  serve  as  a 
basis  for  estimating  the  economical  advantage  of  employing 
electricity  instead  of  gas  to  heat  ordinary  baths. 

Bath  No.  1  was  like  that  shown  in  Fig.  78.  The  stove  was 
of  the  kind  represented  in  Fig.  76.  The  temperature  to  which 
the  bath  was  tested  was  250°,  which  is  as  high  as  copper  should 
be  heated.  The  temperatures  given  in  the  first  column  are  the 
temperatures  of  the  copper  bath,  while  those  in  the  second 
column  are  the  temperatures  of  the  upper  compartment.  The 
'third  column  gives  the  current ;  the  fourth,  its  fall  of  potential 
within  the  bath ;  the  fifth,  the  number  of  watts  consumed  ;  and 
the  sixth,  the  number  of  hours  which  a  kilowatt  will  maintain 
the  temperatures  recorded  in  the  first  and  second  columns. 


Bath  No.  1 


Temperature 
in  Copper 
Bath 

Temperature 
in  Upper 
Compartment 

CURRENT 

FALL  IN 
POTENTIAL 

WATTS 

Hours 
per  Kilo- 
watt 

100° 

48° 

1.73  amp. 

15.9  volts 

27.5 

36.36 

125 

58 

2.10 

18.8 

39.48 

25.58 

150 

65 

2.50 

21.4 

53.50 

18.69 

175 

72 

2.77 

23.2 

64.26 

15.56 

200 

85 

3.16 

25.5 

80.58 

12.41 

225 

96 

3.48 

27.0 

93.96 

10.75 

250 

.     106 

4.07 

29.2 

118.84 

8.41 

Bath  No.  2  differed  from  No.  1  only  in  that  a  stove  of  the 
kind  represented  in  Fig.  77  was  used.  It  was  tested  between 
150°  and  258°. 


LABORATORY  ELECTRIC  HEATING  APPLIANCES      529 


Bath  No.  2 


Temperature 
in  Copper 
Bath 

Temperature 
in  Upper 
Compartment 

CURRENT 

FALL  IN 
POTENTIAL 

WATTS 

Hours 
per  Kilo- 
watt 

150° 
175 

73.0° 
83.5 

1.20  amp. 
1.39     « 

40.3  volts 
44.5     " 

48.36 
61.85 

20.67 
16.10 

200 

95.0 

1.61     " 

49.0     " 

78.89 

12.60 

225 

104.5 

1.89     " 

51.5     M 

97.33 

10.20 

258 

111.0 

2.53    " 

50.0     " 

126.50 

7.90 

An  even  distribution  of  the  conducting  material  over  the 
surface  of  the  soapstone  is  of  great  importance;  for,  if  it  is 
unevenly  distributed,  the  resistance  in  different  regions  will 
vary  and  more  heat  may  be  developed  at  some  points  than  at 
others.  The  result,  in  the  case  of  a  carelessly  prepared  stove, 
is  that  the  graphite  burns  out  in  certain  spots,  while  the  heat 
developed  upon  the  whole  surface  is  much  less  than  it  ought 
to  bear  with  safety.  The  process  of  applying  the  graphite,  or, 
rather,  the  stove  polish,  is  given  in  some  detail. 

The  iron  plates  at  the  ends  of  the  soapstone  block  (Fig.  76), 
or  of  the  strips  (Fig.  77),  are  removed  and  their  inner  surfaces, 
and  also  the  portions  of  soapstone  which  are  covered  by  the 
plates,  are  evenly  spread  with  a  rather  thick  layer  of  the  paste. 
The  plates  are  then  replaced  and  bolted  together.  The  block, 
or  strip  (connected  in  the  circuit  in  which  there  is  also  an 
ammeter),  is  placed  on  a  piece  of  asbestus  board  or  of  some 
other  nonconducting  material  which  is  heated  over  a  lamp. 
When  the  stone  is  hot  enough  to  evaporate  water  with  rapidity 
but  without  "  sputtering,"  the  paste,  much  thinned  with  water, 
is  evenly  applied  with  a  flat  camel- s-hair  brush  whose  width  is 
about  25  mm.  The  whole  stone  should  be  covered  each  time, 
but  the  brush  should  not  pass  more  than  once  over  any  one 
spot.  After  each  application  the  surface  is  vigorously  polished 
with  a  stiff  toothbrush.  As  soon  as  the  conductivity  of  the 


530  QUANTITATIVE  EXERCISES 

graphite  covering  has  increased  to  a  point  where  the  current 
suffices  to  maintain  the  proper  temperature  of  the  stone,  it  is 
removed  from  the  asbestus  board  and  the  surface  is  carefully 
explored  with  a  voltmeter  with  reference  to  the  uniformity  of 
the  distribution  of  resistance.  The  subsequent  applications  of 
graphite  are  regulated  with  a  view  to  securing  equality  of  con- 
ductivity over  the  whole  surface  ;  that  is,  those  spots  which  are 
found  by  the  voltmeter  to  exhibit  a  higher  resistance  are  painted 
more  thickly  than  the  others.  With  care  and  a  little  experience 
it  becomes  easy  to  prepare  in  a  short  time  a  stove  on  which  the 
graphite  is  very  evenly  distributed  and  so  firmly  adherent  that 
it  will  not  visibly  soil  the  fingers  in  handling.  As  the  painting 
proceeds,  the  surface  is  apt  to  become  so  hot  that  the  water  is 
evaporated  with  explosive  violence,  and  the  surface  presents  a 
spotted  appearance,  owing  to  an  uneven  spreading  of  the  mate- 
rial. When  this  occurs  the  circuit  must  be  broken  and  the 
stone  allowed  to  cool  before  making  an  application. 

In  heating  a  bath  by  electricity  it  is  necessary  always  to  in- 
sert in  the  circuit  a  rheostat  for  the  purpose  of  regulating  the 
temperature  of  the  bath,  and,  since  all  of  the  heat  generated  in 
the  rheostat  is  wasted,  it  is  important  to  reduce  to  a  minimum 
the  amount  of  resistance  so  employed.  This  can  be  done  by 
regulating  the  thickness  of  the  graphite  covering  of  the  stove 
with  reference  to  the  voltage  of  the  current  which  is  to  be  used. 
In  other  words,  the  stove  should  use  up,  as  nearly  as  may  be, 
the  whole  energy  of  the  current  in  maintaining  the  required 
temperature  within  the  bath.  For  this  reason  it  would  be  better 
to  have  a  stove  for  each  temperature  which  it  is  desired  to 
keep,  e.g.  one  for  a  temperature  of  100°  or  110°,  another  for 
125°,  still  another  for  150°,  etc.,  each  utilizing  nearly  the  whole 
electromotive  force  in  keeping  the  temperature  for  which  it  was 
designed.  Considerable  latitude  in  this  matter  is  secured  by 
using  stoves  like  that  shown  in  Fig.  77,  where  the  various  pairs 
of  plates  may  be  used  in  parallel  or  in  series,  according  to  the 
temperature  to  be  maintained  and  the  electromotive  force  of  the 


LABORATORY  ELECTRIC   HEATING  APPLIANCES      531 


available  source  of  current.  In  preparing  the  stove  of  this  kind 
to  which  the  data  given  under  "  Bath  No.  2  "  relate,  the  appli- 
cation of  the  graphite  to  each  strip  was  continued  until,  with  a 
fall  of  potential  of  50  volts,  it  conducted  one  ampere  of  current. 
The  temperature  of  the  stone  at  the  time  of  finishing  the  strips 
was  not,  of  course,  always  the  same,  so  that  the  resistance  at  any 
given  temperature  varied  somewhat  from  strip  to  strip,  but  not 
sufficiently  to  lead  to  difficulties. 

Before  using  a  new  graphite  stove  in  its  bath  for  the  purpose 
of  maintaining  constant  temperatures  it  should  be  made  to  keep 
in  the  same  bath  for  several  hours  a  somewhat  higher  tempera- 
ture than  the  highest  for  which  it  was  designed.  The  resistance 
at  any  given  lower  temperature  is  thereafter  fairly  constant. 
In  any  subsequent  use  of  the  stove  the  current  should  never  be 
allowed  to  exceed  that  which  was  employed  in  the  preliminary 
heating. 

3.  TUBE  FURNACE 

Figure  79  shows  a  cross  section  of  a  furnace  for  heating  sub- 
stances inclosed  in  tubes  (Carius  tubes,  etc.).  a  is  a  wooden 
box  625  mm.  in  length  which 
is  so  lined  and  covered  with 
asbestus  board  as  to  give  the 
confined  air  spaces  designated 
by  b.  c  is  a  porcelain  tube  hav- 
ing the  length  of  the  box  and 
an  internal  diameter  of  30  mm. 
The  space  d  around  c  is  suffi- 
cient to  accommodate  larger 
tubes  when  required  to  do  so. 
An  iron  tube  of  thin  wall  is 

slipped  into  c  to  aid  in  distributing  the  heat  and  to  prevent 
injury  to  the  porcelain  tube  when  explosions  occur. 

Figure  80  shows  an  iron  collar  which  is  fastened  to  each  end 
of  the  porcelain  tube  and  serves  for  the  attachment  of  the  wires. 


FIG.  79 


532 


QUANTITATIVE  EXERCISES 


The  graphite  (stove  polish)  is  applied  to  the  outside  of  the 
porcelain  tube  in  the  same  manner  as  to  the  soapstone  sur- 
faces, care  being  taken  that  the  fall  in  potential  shall  be  nearly 
the  same  for  equal  distances  in  different  parts  of  the  tube.  The 
paste  is  applied  somewhat  thickly  under 
the  iron  rings.  The  following  table  gives 
the  data  which  were  obtained  in  testing  a 
furnace  of  the  kind  where  a  covering  of 
graphite  mixed  with  soap  instead  of  the  usual 
stove-polish  paste  was  employed.  In  pre- 
paring the  tube  a  quantity  of  finely  pulver- 
ized electrotyping  graphite  was  made  into 
a  thick  paste  by  mixing  it  with  a  small  quantity  of  a  concen- 
trated solution  of  hard  soap.  This  was  applied  in  the  manner 
previously  described. 

Porcelain  Tube 


FIG.  80 


TEMPERA- 
TURE 

CURRENT 

FALL,  OF 
POTENTIAL 

WATTS 

HOURS  PER 
KILOWATT 

150° 

1.54  amp. 

28.5  volts 

43.89 

22.8 

175 

1.74    " 

31.7 

55.66 

17.9 

200 

1.96    " 

35.6 

69.77 

14.3 

225 

2.07     " 

37.4 

77.42 

12.9 

250 

2.34    " 

42.0 

98.28 

10.2 

275 

2.59     « 

45.0 

116.55 

8.6 

300 

2.79    " 

49.2 

137.27 

7.3 

355 

3.38    " 

65.3 

186.91 

5.3 

It  is  important  for  users  of  graphite  baths  to  know  approxi- 
mately what  temperature  a  graphite  surface  will  bear  without 
injury.  It  is  unfortunate  that  an  admixture  of  clay  is  necessary 
in  order  to  improve  its  spreading  and  adhesive  qualities,  for  the 
clay  begins  to  lose  its  water  at  about  425°,  and  at  this  point  a 
change  in  the  resistance  of  the  material  occurs.  Speaking  gen- 
erally, it  may  be  stated  that  the  temperature  of  the  stove  itself 
should  not  be  allowed  to  rise  above  400°.  How  nearly  the 


LABORATORY  ELECTRIC  HEATING  APPLIANCES      533 

temperature  of  a  bath  may  be  made  to  approach  this  limit  will 
depend,  of  course,  upon  the  volume  of  the  space  to  be  heated  as 
compared  with  the  area  of  the  heating  surface.  In  small  baths 
with  relatively  large  heating  surface  a  constant  temperature  of 
400°  may  be  maintained,  but  there  is  danger  that  in  attempting 
to  reach  so  high  a  temperature  too  quickly  the  clay  will  become 
overheated  and  lose  some  of  its  water.  It  has  been  found  safe 
and  practicable,  with  properly  prepared  stoves  having  a  suitable 
area  of  heating  surface,  to  maintain,  in  a  considerable  variety  of 
baths,  any  temperature  up  to  350° ;  and,  though  higher  tem- 
peratures may  be  secured,  it  is  at  some  risk  to  the  stove.  The 
principal  danger  lies  in  using  too  large  a  current  while  raising 
the  temperature  of  the  bath. 

4.  PLATINUM  WIRE  STOVES 

For  baths  in  which  higher  temperatures  are  to  be  kept  than 
can  safely  be  produced  by  graphite  stoves,  the  arrangement 
shown  in  Fig.  81  is  to  be  recommended.  In  this  the  resistance 
of  a  small  platinum  wire  is  utilized  to  generate  the  heat,  a  is 
a  soapstone  block,  of  any  size  to  suit  the  dimensions  of  the  bath, 
with  bolts  near 
two  opposite 
sides,  which 
serve  to  fix  the 
coils  in  their 
places  and  for 
the  attachment 

of  the  wires  of 

FIG.  81 

the  circuit.   Four 

of  the  bolt  ends  are  covered  with  the  soapstone  caps  b.  The 
purpose  of  the  caps  will  be  made  apparent  by  an  inspection  of 
Fig.  78.  The  length  and  size  of  the  platinum  wire  are  made 
to  suit  the  conditions  which  are  to  be  met,  i.e.  the  size  of  the 
bath,  the  temperature  to  which  it  is  to  be  heated,  the  voltage 


534 


QUANTITATIVE  EXERCISES 


of  the  available  source  of  current,  etc.  In  the  furnace  shown 
in  the  figure  the  wire  is  No.  32  (B.  &  S.  gauge)  and  eight  feet 
long. 

To  prepare  the  wire  it  is  straightened,  heated  to  redness 
bj  the  current,  and  then  wound  on  a  threaded  steel  rod  of  the 
proper  diameter.  Such  furnaces  may,  of  course,  be  used  in  the 
place  of  the  graphite  stoves  previously  described,  and  the  fol- 
lowing table  gives  the  data  which  were  obtained  while  testing 
one  of  them  in  a  bath  arranged  as  shown  in  Fig.  78,  except  that 
the  shelf  /  in  this  case  was  perforated  so  as  to  permit  free  cir- 
culation of  air  between  the  upper  and  lower  compartments. 

Bath  with  Platinum  Wire  Stove 


Temperature 
in  Copper 
Bath 

Temperature 
in  Upper     . 
Compartment 

CURRENT 

FALL  OF 
POTENTIAL 

WATTS 

Hours 
per  Kilo- 
watt 

100° 

1.08  amp. 

28.5  volts 

30.8 

30.5 

125 

72° 

1.24 

34.9 

43.3 

23.1 

150 

80 

1.4 

41.8 

58.5 

17.1 

175 

89 

l.C 

51.0 

81.6 

12.3 

200 

110 

1.77 

59.8 

105.9 

9.5 

225 

125 

1.89 

66.4 

125.5 

8.0 

250 

144 

2.00 

73.0 

146.0 

6.9 

Figure  82  represents  a  bath  with  platinum  wire  stove  which 
was  constructed  for  the  purpose  of  heating  rather  large  vessels 
to  any  temperature  up  to  500°.  The  heating  coils  of  the  stove 
a  contained  about  16  feet  of  No.  26  platinum  wire  ;  b  is  a  terra- 
cotta cylinder  like  those  used  in  chimneys  for  stovepipes  ;  c  is 
a  sheet-iron  cylinder  which  is  covered  inside  and  outside  with 
asbestus  paper;  d  and  d  are  covers  of  asbestus  board  ;  e  is  a 
terra-cotta  receptacle  such  as  is  used  for  flowerpots ;  /  is  sand 
in  which  the  stove  and  the  lower  ends  of  b  and  c  are  imbedded. 
The  dimensions  of  the  free  heated  space  within  the  bath  are 
250  and  175  mm.  For  the  sake  of  cleanliness  and  of  improved 


LABORATORY  ELECTRIC   HEATING  APPLIANCES      535 

appearance,  e,  c,  d,  and  the  outside  of  b  are  covered  with  alu- 
minium paint.  With  temperatures  within,  up  to  300°,  it  is 
safe  to  surround  the  outer  cylinder  c  with  woolen  material, 


or,  better,  with  a  thick  pad  made  of  hair.  When  higher  tem- 
peratures are  to  be  maintained,  there  is  substituted  for  the 
pad  a  wooden  cylinder  with  an  internal  diameter  equal  to  the 
extreme  width  of  the  base  e,  and  this  is  covered  with  a  strip 
of  woolen  cloth.  The  test  of  this  bath  up  to  400°,  with  the 


536 


QUANTITATIVE  EXERCISES 


thermometer  bulb  located  in  the  center  of  the  heated  space, 
gave  the  following  data. 


TEMPERA- 
TURE 

CURRENT 

FALL,  OF 
POTENTIAL 

WATTS 

HOURS  PER 
KILOWATT 

100° 

2.08  amp. 

20.7  volts 

43.06 

23.22 

125 

2.20     " 

23.0     « 

50.60 

19.76 

150 

2.45     " 

28.0     * 

68.60 

14.58 

175 

2.70     " 

32.3     ' 

87.21 

11.47 

200 

2.85     " 

35.7     ' 

100.75 

9.92 

225 

3.04     " 

40.8     « 

124.03 

8.06 

250 

3.09     " 

43.0     « 

132.87 

7.53 

275 

3.29     " 

48.5     ' 

159.57 

6.27 

300 

3.50     » 

53.5     « 

177.25 

5.64 

350 

3.73     " 

61.0     « 

227.53 

4.40 

400 

4.00     " 

68.0     « 

272.00 

3.67 

The  bath  was  not  tested  beyond  400°  except  to  determine 
that,  with  a  current  pressure  of  110  volts,  its  temperature  rises 
above  500°. 

Arrangements  like  that  just  described  are  an  effective  sub- 
stitute for  all  of  the  oil  and  paraffin  baths  which  give  so  much 
offense  in  the  laboratory,  and,  like  all  electrically  heated  baths, 
they  have  the  great  advantage  that,  with  a  current  of  constant 
voltage,  they  can  be  regulated  with  a  high  degree  of  precision. 


CHAPTER  XXIV 

AN  ELECTRICAL  METHOD  FOR  THE  COMBUSTION  OF  ORGANIC 

COMPOUNDS 

The  process  for  the  analysis  of  organic  compounds  which  is 
here  described  is  applicable  to  the  combustion  of  hydrocarbons 
and  their  oxygen,  halogen,  nitrogen,  and  sulphur  derivatives. 
The  advantages  which  it  has  been  found  to  possess  over  the 
usual  method  are  the  following  :  (1)  The  apparatus  required  is 
so  compact  and  of  such  small  dimensions  that  the  analyses  can 
be  made  at  the  working  table  of  the  student ;  (2)  the  waste  of 
heat  energy  is  much  smaller;  and  (3)  the  time  consumed  in 
preparing  for  and  in  making  a  combustion  is  much  less  than 
by  the  older  process. 

Figure  83  represents  the  apparatus  in  its  simplest  form.  The 
glass  combustion  tube  a  is  closed  at  one  end  and  has  a  length 
of  350  mm.  and  an  internal  diameter  of  15  mm.  Through  the 
rubber  stopper  in  its  open  end  there  pass  (1)  the  porcelain 
tube  c  which  has  a  length  of  250  mm.  and  a  diameter  of  6  mm.; 
(2)  the  glass  tube  &,  through  which  the  products  of  combustion 
enter  the  absorption  apparatus;  and  (3)  a  rather  stout  platinum 
wire  (No.  18  B.  &  S.  gauge),  which  extends  from  /  to  j.  The 
porcelain  tube  c  is  joined,  outside  of  the  stopper,  by  means  of 
rubber  tubing,  to  the  branched  glass  tube  d.  The  latter  is  pro- 
vided with  a  stopper  through  which  passes  the  platinum  wire  g 
(also  No.  18),  which  extends  into  the  porcelain  tube  to  the  point 

A,  where  it  is  joined  to  a  smaller  platinum  wire  (about  No.  29 

B.  &  S.  gauge).    The  smaller  wire  has  a  length  of  about  1 1  meters 
and  weighs  approximately  2J  grams.    It  extends  from  its  junction 
with  the  larger  wire  at  h  through  the  porcelain  tube  to  the  inner 
end  of  the  latter,  and  then  returns  on  the  outside  in  a  series  of 

537 


538 


QUANTITATIVE  EXERCISES 


suspended  coils  to  the  point  /,  where  it  joins  the  larger  wire  /. 
Thicker  wire  is  used  from  /  to  j  and  from  e  to  h  in  order  to  avoid 

any  overheating  of  the  rubber  stopper 
by  the  current.  The  roll  of  copper 
wire  gauze  5,  about  60  mm.  in  length, 
is  inserted  between  the  end  of  the 
porcelain  tube  and  the  boat  containing 
the  substance  to  be  burned. 

The  oxygen  or  air  which  is  to  be 
used  in  the  combustion  enters  the 
apparatus  at  d,  and  while  passing  over 
the  portion  of  the  small  wire  which 
is  within  the  porcelain  tube  has  its 
temperature  raised  more  or  less  ac- 
cording to  the  rate  of  its  flow.  It  is 
therefore  already  hot  when  it  enters 
the  tube  £,  where  the  combustion  is 
to  be  effected.  The  excess  of  the 
oxygen  and  the  products  of  the  com- 
bustion of  the  substance  pass  together 
over  the  heated  coils  on  the  outside, 
completing  the  burning  of  any  unox- 
idized  material  coming  from  the  rear. 
In  the  earlier  work  with  this  appa- 
ratus, stems  of  long  clay  tobacco  pipes 
were  employed;  but  owing  to  the 
highly  porous  character  of  this  mate- 
rial it  was  found  somewhat  difficult  to 
obtain  satisfactory  determinations  of 
hydrogen.  These  stems  were  there- 
fore discarded  for  porcelain  tubes, 
which  were  made  for  the  author  at  the 
Royal  Berlin  Porcelain  Works.  The 
latter  have  proved  exceedingly  dur- 
able, and  they  are  not,  of  course,  sensibly  hygroscopic. 


ELECTRICAL   METHOD  FOR  COMBUSTION  539 

The  roll  of  copper  wire  gauze  b  is  not  absolutely  necessary 
when  the  substances  to  be  burned  do  not  readily  yield  explosive 
mixtures  with  oxygen.  Its  presence  is,  however,  of  advantage 
because  much  less  care  is  required  in  the  management  of  the 
combustion  with  it  than  without  it.  If  the  substances  are 
liquids  or  if  they  readily  yield  large  quantities  of  inflammable 
vapors  when  heated,  the  roll  of  wire  gauze  must  be  inserted 
between  the  material  and  the  end  of  the  porcelain  tube  through 
which  the  oxygen  enters.  The  objection  to  the  use  of  the  roll 
of  wire  gauze  is  that  the  flow  of  oxygen  or  air  must  be  con- 
tinued for  some  time  after  the  completion  of  the  combustion  in 
order  to  insure  the  removal  of  all  of  the  carbon  dioxide  and 
water  from  the  space  occupied  by  the  boat. 

The  combustion  is  conducted  in  the  following  manner :  Hav- 
ing placed  in  the  positions  indicated  in  the  figure  the  boat  con- 
taining the  material  and  the  roll  of  copper  wire  gauze  (which  in 
the  beginning  may  or  may  not  be  oxidized),  and  having  joined 
the  tube  k  to  the  usual  train  of  absorption  apparatuses,  a  slow 
current  of  dry  and  purified  oxygen  is  admitted  and  the  electric 
current  is  closed  through  a  regulating  rheostat.  The  flow  of 
the  current  is  gradually  increased  until  the  wire  coils  assume  a 
bright  red  color,  when  the  roll  of  gauze  b  is  heated  by  means  of 
a  gas  lamp  giving  a  broad  thin  flame  to  a  temperature  at  which 
the  copper  is  readily  oxidized  by  the  hot  oxygen  issuing  from 
the  porcelain  tube.  The  substance  in  the  boat  is  then  heated 
with  the  same  lamp,  or  with  another  which  is  held  in  the  hand. 
The  rate  of  heating  and  the  flow  of  oxygen  are  so  regulated 
with  respect  to  each  other  that  at  least  one-half  of  the  roll  of 
wire  gauze  is  kep^  in  the  oxidized  condition  during  the  entire  com- 
bustion. After  the  formation  of  volatile  products  has  ceased, 
the  reoxidation  of  the  copper  progresses  rapidly  and  the  oxygen 
enters  the  rear  compartment,  burning  any  residue  of  carbon  in 
the  boat  or  upon  the  glass.  Having  finished  the  combustion 
of  the  substance,  the  current  of  oxygen  is  replaced  by  one 
of  dried  and  purified  air,  and  the  flow  of  the  latter  must,  of 


540  QUANTITATIVE  EXERCISES 

course,  be  continued  until  the  products  of  combustion  have  all 
been  expelled  from  the  space  behind  the  roll  of  wire  gauze.  It 
is  here  that  a  miscalculation  is  likely  to  be  made.  The  time 
required  for  the  complete  removal  of  the  products  depends 
principally  upon  the  freedom  of  diffusion  through  the  gauze, 
and  for  this  reason  it  should  not  be  rolled  too  tightly. 

The  apparatus  as  already  described  is  adapted  to  the  com- 
bustion of  those  solids  and  liquids  which  consist  of  carbon  and 
hydrogen  or  of  carbon,  hydrogen,  and  oxygen. 

The  heating  of  the  roll  of  wire  gauze  6,  and  at  times  of  the 
substance  also,  is  facilitated  by  inverting  over  the  tube,  at  a 
little  distance  above  it,  a  trough  made  of  asbestus  board,  the 
side  of  the  trough  at  the  back  being  much  deeper  than  that  in 
front.  This  arrangement  is  supported  in  its  position  by  a  rod 
which  is  inserted  in  a  heavy  block  resting  upon  the  worktable 
behind  the  tube.  The  device  is  also  of  advantage  in  protecting 
the  tube  from  draughts  of  cold  air  during  the  combustion  and 
during  the  subsequent  cooling  period.  The  portion  of  the  glass 
tube  which  is  occupied  by  the  porcelain  tube  and  the  platinum 
wire  is  protected  on  the  bottom  by  a  semicircular  strip  of 
asbestus  board  which  is  inserted  in  the  clamp  between  the  lower 
jaw  and  the  glass.  To  protect  the  upper  portion  of  the  tube  in 
the  same  region,  a  semicircular  trough  of  mica  is  inverted  over 
it  behind  the  clamp  in  such  a  manner  that  the  lower  edges  of 
the  mica  rest  in  the  asbestus  trough  below.  The  mica  is  made 
to  keep  its  curved  form  by  riveting  it  to  narrow  strips  of  metal 
and  then  bending  the  latter  to  the  required  shape. 

The  cooling  of  the  tube  requires  some  care.  The  current 
should  be  reduced  quite  gradually,  and  it  is  well  also,  as  soon 
as  the  combustion  is  finished,  to  cover  the  portion  of  the  glass 
tube  which  is  beyond  the  porcelain  one  with  the  soot  from  a 
smoky  flame,  and  to  take  any  other  measures  for  the  protection 
of  the  tube  which  will  contribute  toward  the  proper  annealing 
of  the  glass.  Care  must  likewise  be  taken  never  to  allow  the 
platinum  loops  to  come  in  contact  with  the  glass  either  while 


ELECTRICAL  METHOD  FOR  COMBUSTION  541 

heating  or  cooling  the  tube,  since  in  the  former  case  the  metal 
is  likely  to  stick  to  the  glass,  while  in  the  latter  it  is  quite  sure 
to  crack  at  some  lower  temperature.  If  the  loops  are  too  large, 
the  wire  should  be  taken  out  and  rewound. 

The  time  required  for  the  combustion  does  not  ordinarily 
exceed  half  an  hour,  and  it  may  be  reduced  to  twenty  or  even 
fifteen  minutes  if  the  substance  to  be  burned  is  of  such  a  char- 
acter that  the  roll  of  wire  gauze  can  be  dispensed  with.  Its 
omission  is,  however,  not  recommended  except  to  those  who 
have  had  some  experience  with  the  method.  It  is  well  to  place 
at  the  end  of  the  absorption  flame  a  Geissler  apparatus  partly 
filled  with  a  very  dilute  solution  of  palladium  chloride  in  order 
to  test  the  completeness  of  the  combustion.  If  it  is  incomplete, 
the  fact  will  be  made  apparent  by  the  dark  color  developed  in 
the  solution  of  the  palladium  salt. 

The  consumption  of  electrical  energy  is  small,  amounting  to 
about  3.7  amperes  at  54  volts  (194.4  watts,  or  about  one-fourth 
of  a  horse  power)  during  the  time  when  the  highest  temperature 
is  maintained. 

At  the  highest  temperature  employed  during  the  combustion 
(at  a  bright  red,  but  not  a  white  heat),  especially  while  the  wire 
is  new,  there  is  a  sensible  volatilization  of  platinum.  This 
volatility  of  the  metal  in  an  atmosphere  of  oxygen,  at  compara- 
tively moderate  temperatures,  has  been  repeatedly  observed.  In 
one  case  a  platinum  wire  weighing  in  the  beginning  2.3446 
grams  was  found  at  the  end  of  the  thirty-eighth  combustion  to 
have  lost  0.1337  gram  in  weight.  The  volatilized  metal  settles 
upon  the  surfaces  of  the  glass  and  porcelain  tubes  as  a  dark 
deposit  which  at  first  may  be  mistaken  for  carbon.  The  pres- 
ence of  such  films  of  volatilized  platinum  upon  the  inner 
surface  of  the  tube  is,  of  course,  of  some  advantage  in  the 
combustion. 

There  are  certain  obvious  objections  to  the  use  of  the  short 
closed  combustion  tube  represented  in  Fig.  83.  In  the  first 
place,  whenever  the  boat  is  to  be  introduced  or  withdrawn,  it  is 


542 


QUANTITATIVE  EXERCISES 


necessary  to  remove  and  reinsert  the  rubber  stopper  with  the 
tubes  and  wires  which  pass  through  it,  and  also  the  roll  of  wire 
gauze;  and  during  such  manipulation 
more  or  less  atmospheric  moisture  is  ad- 
mitted, and  the  wire  loops,  which  should 
remain  distributed  uniformly  along  the 
porcelain  tube,  are  easily  displaced  and 
may  become  entangled  one  with  another. 
Again,  as  previously  mentioned,  the  flow 
of  air  must  be  continued  for  some  time 
after  the  combustion  is  otherwise  finished, 
in  order  to  insure  the  complete  removal  of 
carbon  dioxide  and  water  from  the  space 
occupied  by  the  boat.  These  difficulties 
are  wholly  obviated  by  using  a  somewhat 
longer  tube  which  is  open  at  both  ends,  as 
represented  in  Fig.  84.  In  this  arrange- 
ment the  boat  is  introduced  from  the  rear, 
and  there  is  placed  behind  it  a  second 
short  roll  of  copper  wire  gauze.  The 
stopper  in  the  front  end  of  the  combus- 
tion tube,  the  forward  roll  of  wire  gauze, 
and  also  the  apparatus  as  a  whole  are 
never  disturbed.  Each  roll  of  wire  gauze 
is  heated  by  a  lamp  giving  a  broad  thin 
flame,  and  there  is  inverted  over  both 
rolls  and  the  space  between  them  the 
asbestus  shield  already  described.  The 
lamps  should  be  raised  until  the  bottom 
of  the  tube  is  just  within  the  blue  re'gion 
of  the  flames.  In  this  way  the  overheat- 
ing of  the  tube  at  certain  points  is  effec- 
tually avoided. 

To  prevent  any  sagging  of  the  com- 
bustion tube  while  hot,  it  is  supported  at  a  point  just  beneath 


ELECTRICAL  METHOD  FOR  COMBUSTION  543 

the  end  of  the  porcelain  tube  by  a  forked  or  notched  standard, 
which  is  placed  under  the  asbestus  trough  in  which  the  front 
portion  of  the  apparatus  lies. 

The  combustion  is  conducted  in  much  the  same  manner  as 
in  the  short  closed  tube,  except  that  in  addition  to  the  oxygen 
which  enters  the  front  of  the  tube  a,  slow  current  of  air  or 
oxygen  is  admitted  from  the  rear  during  the  entire  experiment. 
This  prevents  any  accumulation  of  volatilized  matter  in  the 
back  part  of  the  tube  and  aids  in  the  expulsion  of  the  products 
of  combustion  from  the  space  occupied  by  the  boat.  If  the  sub- 
stance to  be  burned  is  very  volatile,  it  is  advisable  to  introduce 
air  and  not  oxygen  at  the  rear,  and  to  employ  behind  the  boat 
a  roll  of  gauze  which  is  only  partially  oxidized.  In  this  way 
the  vapors  of  the  substance  may  be  diluted  with  nitrogen  to  any 
desired  extent.  In  order  to  avoid  any  bending  of  the  tube  in 
consequence  of  its  increased  length  while  hot,  the  clamp  at  the 
rear  is  employed  only  as  a  support  and  is  not  allowed  to  grip 
the  tube  so  tightly  as  to  interfere  with  its  freedom  of  movement 
back  and  forth. 

THE  COMBUSTION  OF   SUBSTANCES  CONTAINING 
NITKOGEN 

For  the  determination  of  carbon  and  hydrogen  in  compounds 
containing  nitrogen  there  is  placed  in  the  combustion  tube 
(1)  a  roll,  100  mm.  in  length,  of  copper  wire  gauze  which  has 
been  reduced  in  the  usual  way  by  methyl  alcohol ;  (2)  a  roll 
80  mm.  in  length,  of  wire  gauze  which  has  been  well  oxidized; 
(3)  the  boat  containing  the  substance;  and  (4)  a  short  roll  of 
wire  gauze  also  well  oxidized. 

During  the  combustion  each  of  the  three  rolls  is  heated  by  a 
burner  with  a  broad  thin  flame,  the  last  lamp  serving  also  for 
heating  the  substance,  and  the  portion  of  the  tube  occupied  by 
the  copper  is  covered  with  a  screen  of  asbestus  board  to  insure 
a  sufficiently  high  temperature  for  the  reduction  of  nitric  oxide. 


544  QUANTITATIVE  EXERCISES 

The  flow  of  oxygen  through  the  porcelain  tube  is  so  regulated 
that  only  about  one  quarter  of  the  copper  roll  (1)  is  oxidized, 
while  at  the  rear  it  is  admitted  as  rapidly  as  may  be  necessary 
to  keep  a  portion  of  the  second  roll  (2)  at  all  times  in  an 
oxidized  condition. 

In  the  case  of  nitro-derivatives  it  is  well  to  introduce  into  the 
boat  with  the  substance  a  quantity  of  loose  copper  oxide  or  of 
finely  divided  silica. 

The  combustion  of  nitrogenous  compounds  must  be  con- 
ducted much  more  cautiously  and  slowly  than  that  of  other 
substances,  owing  to  the  necessity  of  burning  the  carbon  com- 
pletely before  the  products  of  the  combustion  reach  the  reduced 
roll  of  wire  gauze.  If  the  combustion  of  the  carbon  is  incom- 
plete, —  that  is,  if  a  portion  of  it  is  finished  in  the  front  end  of 
the  tube,  —  there  is  danger  that  some  of  the  nitric  oxide  will 
escape  reduction. 

THE  COMBUSTION  OF  HALOGEN  COMPOUNDS 

To  prepare  the  apparatus  for  the  analysis  of  substances  con- 
taining the  halogens,  a  piece  of  silver  foil  about  50  mm.  in 
width  is  rolled  up  in  a  sheet  of  thick  paper,  which  is  afterwards 
withdrawn.  The  silver  roll  is  placed  in  the  tube  quite  close  to 
the  end  of  the  porcelain  tube  and  is  not  directly  heated  during 
the  combustion.  In  other  respects  the  arrangements  are  the 
same  as  for  the  combustion  of  non-nitrogenous  compounds.  A 
roll  of  well-oxidized  copper  wire  gauze  follows  the  one  of  silver, 
then  the  boat  containing  the  substance,  and  finally  a  second 
roll  of  oxidized  copper  wire  gauze. 

During  the  combustion  there  is  formed  a  quantity  of  fusible 
cuprous-halogen  salt,  which  deposits  itself  more  or  less  upon 
the  inner  surface  of  the  glass  tube,  but  does  not  at  any  time  get 
beyond  the  roll  of  silver  foil  into  the  space  occupied  by  the 
porcelain  tube  and  platinum  wire.  On  cooling,  the  cuprous- 
halogen  salt,  in  accordance  with  the  well-known  behavior  of  such 


ELECTRICAL  METHOD  FOR  COMBUSTION  545 

compounds,  absorbs  large  quantities  of  oxygen,  only  to  give  it 
up  again  when  the  apparatus  is  reheated  in  a  succeeding  experi- 
ment. At  the  same  time  the  copper  wire  in  the  oxidized  rolls 
grows  thinner  and  becomes  quite  brittle. 

The  quantity  of  cuprous  salt  accumulates  after  a  few  combus- 
tions to  such  an  extent  that  the  time  required  for  its  oxidation 
is  considerable.  Hence  it  is  well  frequently  to  cleanse  the  com- 
bustion tube  and  to  renew  at  the  same  time  the  rolls  of  copper 
wire  gauze. 

THE  COMBUSTION  OF  SULPHUR  COMPOUNDS 

The  determination  of  carbon  and  hydrogen  in  compounds 
containing  sulphur  is  quite  simple.  The  only  change  which  it 
is  necessary  to  make  in  the  arrangement  for  non-nitrogenous  and 
non-halogen  compounds,  in  order  to  adapt  the  method  to  the 
combustion  of  sulphur  compounds,  is  to  substitute  lead  chro- 
mate  for  the  oxidized  copper  wire  gauze  which  is  nearest  to  the 
end  of  the  porcelain  tube.  Instead  of  maintaining  the  lead 
chromate  in  its  position  in  the  tube  by  means  of  plugs  of 
asbestus  or  of  wire  gauze,  it  is  more  convenient  and  better  for 
the  glass  tube  to  introduce  it  in  the  form  of  a  cartridge.  This 
is  prepared  by  filling  with  the  loose  granular  chromate  a  shell 
made  from  fine  copper  wire  gauze. 


TABLE  OF  INTERNATIONAL  ATOMIC  WEIGHTS 


0=16 

H=l 

O=16 

H=l 

Aluminium  . 

.  Al 

27.1 

26.9 

Neodymium    . 

Nd 

143.6 

142.5 

Antimony     . 

.  Sb 

120.2 

119.3 

Neon   

Ne 

20. 

19.9 

Argon     .  .  . 

.  A 

39.9 

39.6 

Nickel    .  .  .  . 

Ni 

58.7 

58.3 

Arsenic  .  .   . 

.  As 

75.0 

74.4 

Nitrogen    .  .  . 

N 

14.04 

13.93 

Barium  .  .  . 

.  Ba 

137.4 

136.4 

Osmium    .  .  . 

Os 

191. 

189.6 

Bismuth    .  . 

.  Bi 

208.5 

206.9 

Oxygen  .... 

0 

16.00 

15.88 

Boron     .  .  . 

.  B 

11. 

10.9 

Palladium    .  . 

Pd 

106.5 

105.7 

Bromine    .  . 

.  Br 

79.96 

79.36 

Phosphorus     . 

P 

31.0 

30.77 

Cadmium  .  . 

.  Cd 

112.4 

111.6 

Platinum  .  .  . 

Pt 

194.8 

193.3 

Caesium    .  . 

.  Cs 

132.9 

131.9 

Potassium    .  . 

K 

39.15 

38.85 

Calcium    .  . 

.  Ca 

40.1 

39.7 

Praseodymium 

Pr 

140.5 

139.4 

Carbon  .  .  . 

.  C 

12.00 

11.91 

Radium     .  .  . 

Ra 

225. 

223.3 

Cerium  .  .  . 

.  Ce 

140.25 

139.2 

Rhodium  .  .  . 

Rh 

103.0 

102.2 

Chlorine    .   . 

.  Cl 

35.45 

35.18 

Rubidium     .  . 

Rb 

85.5 

84.9 

Chromium   . 

.  Cr 

52.1 

51.7 

Ruthenium  .   . 

Ru 

101.7 

100.9 

Cobalt    .  .  . 

.  Co 

59.0 

58.55 

Samarium    .  . 

Sm 

153. 

149.2 

Columbium 

.    Cb 

94. 

93.3 

Scandium     .  . 

So 

44.1 

43.8 

Copper  ... 

.  Cu 

63.6 

63.1 

Selenium  .  .  . 

Se 

79.2 

78.6 

Erbium  .  .  . 

.  Er 

166. 

164.8 

Silicon    .... 

Si 

28.4 

28.2 

Fluorine    .   . 

.  F 

19. 

18.9 

Silver  . 

A  o* 

107.93 

107.11 

Gadolinium  . 

.  Gd 

150. 

'154.8 

Sodium  .... 

**o 

Na 

23.05 

22.88 

Gallium     .  . 

.  Ga 

70. 

69.5 

Strontium    .  . 

Sr 

87.6 

86.94 

Germanium 

.  Ge 

72.5 

72. 

Sulphur     .  .  . 

S 

32.06 

31.82 

Glucinum 

.  Gl 

9.1 

9.03 

Tantalum     .  . 

Ta 

183. 

181.6 

Gold 

Au 

197.2 

195.7 

Tellurium 

Te 

127.6 

126.6 

Helium  .  .  . 

.  He 

4. 

4. 

Terbium    .  .  . 

Tb 

160. 

158.8 

Hydrogen     . 

.  H 

1.008 

1.000 

Thallium  .  .  . 

Tl 

204.1 

202.6 

Indium  .   .  . 

.  In 

115. 

114.1 

Thorium    .  .   . 

Th 

232.5 

230.8 

Iodine    .  .  . 

.  I 

126.97 

126.01 

Thulium    .  .  . 

Tm 

171. 

169.7 

Iridium      .  . 

.  Ir 

193.0 

191.5 

Tin  

Sn 

119.0 

118.1 

Iron     .  .  .  . 

Fe 

55.9 

55.5 

Titanium  .  .   . 

Ti 

48.1 

47.7 

Krypton    .  . 

.  Kr 

81.8 

81.2 

Tungsten  .  .  . 

W 

184. 

182.6 

lanthanum 

.  La 

138,9 

137.9 

Uranium   .  .  . 

U 

*238.5 

236.7 

Lead    .... 

Pb 

206.9 

205.35 

Vanadium    .  . 

V 

51.2 

50.8 

Lithium     .  . 

.  Li 

7.03 

6.98 

Xenon 

Xe 

128. 

127. 

Magnesium  . 

.  Mg 

24.36 

24.18 

Ytterbium    .  . 

Yb 

173.0 

171.7 

Manganese  . 

.  Mn 

55.0 

54.6 

Yttrium     .  .  . 

Yt 

89.0 

88.3 

Mercury    . 

Hff 

200.0 

198.5 

Zinc           . 

Zn 

65.4 

64.9 

Molybdenum 

AA6 

.  Mo 

96.0 

95.3 

Zirconium    .  . 

Zr 

90.6 

89.9 

546 


INDEX 


Absorption  of  gases  by  liquids,  70. 
Adsorption,  20(3. 

Air,  apparatus  for  purification  of,  353. 
determination  of  carbonic  acid  in, 

332. 

determination  of  oxygen  in,  302. 
displaced  in  weighing,  correction  for, 

22. 

thermometer,  52. 
Albuminoid  ammonia,  determination 

of,  270. 

Alcoholometers,  144. 
Alcohol  thermometers,  54. 
Alkali,   caustic,  determination  of  an 

alkaline  carbonate  in,  415. 
Alkalies,  separation  of  alkaline  earths 

from,  435. 

separation  of  magnesium  from,  438. 
Alkaline  carbonate,  determination  of, 

in  a  caustic  alkali,  415. 
Alkaline  earths,  separation  of,  from 

the  acids,  432. 
Altitude,  correction  of  barometer  for, 

35. 
Aluminium,  separation  of  iron  from, 

400. 
Ammonia,  colorimetric  determination 

of,  270. 

Ammonium  chloride  and  calcium  car- 
bonate, decomposition  of  silicates 

by,  318. 
Ammonium  molybdate,  preparation  of 

solution  of,  283. 

separation  of  phosphoric  acid  by,  298. 
sulphide,   separation  of  phosphoric 

acid  by,  300. 
Ampere,  the,  474. 


Arm  lengths  of  balance,  equality  of,  6. 

Arsenic,  283. 

determination    of,    as    magnesium 
pyroarseniate,  301. 

Arsenious  acid,  iodometric  determina- 
tion of,  229. 

Ashless  paper,  200. 

Atomic  ratios,  calculation  of,  150. 

Atomic  weights,  table  of,  547. 

Balance,  Bunge's,  of  constant  sensi- 
bility, 15. 

description  of,  1. 

determination  of  sensibility  of,  13. 

effect  of  friction  upon  knife  edges 
of,  6. 

equality  of  arm  lengths  of,  6. 

knife  edges  of,  4. 

length  of  beam  of,  5. 

location  of,  6. 

position  of  center  of  gravity  of,  3. 

sensibility  of,  4. 

to  determine  whether  the  arms  are 
equal,  16. 

Verbeck  and  Peckholdt's,  15. 

weight  of  beam  of,  5. 
Barium,  detection  of,  421. 

determination  of,  417. 

determination  of,  as  sulphate,  423. 

determination  of,  as  carbonate,  425. 

determination  of,  as  chromate,  425. 

determination  of,  as  silicofluoride, 
426. 

separation  of,  417. 

separation  of,  from  magnesium,  440. 

separation  of,  from  strontium  and 
calcium,  417. 


547 


548 


QUANTITATIVE  EXERCISES 


Barium    hydroxide     and    carbonate, 
decomposition  of  silicates  by,  317. 
Barometer,  cistern,  30. 

correction  of,  for  altitude,  35. 
correction  of,  for  capillary  depres- 
sion, 35. 

correction  of,  for  latitude,  35. 
correction  of,  for  moisture  in  the  air, 

38. 

correction  of,  for  temperature,  33. 
forms  of,  30. 
Fortin,  30. 
siphon,  31. 

Baths,  air,  with  graphite  stoves,  524. 
hot-air,  194. 
liquid,  197. 
Meyer's,  196. 
Batteries,  storage,  483. 
Beaume's  hydrometer,  141. 
Beckmann  thermometer,  52. 
Beck's  hydrometer,  143. 
Bismuth  oxide,  decomposition  of  sili- 
cates by,  314. 

Boiling  point  of  thermometers,  deter- 
mination of,  43. 
Borax  method  of  determining  water  in 

silicates,  307. 

Boric  acid,  separation  of,  413. 
Borda,  weighing  by  the  method  of,  20. 
Bromine,  determination  of,  213. 
Bumping  of  liquids,  189. 
Bunge's    balance  of   constant   sensi- 
bility, 15. 

Burette,  procedure  in  calibrating,  90. 
Burettes,  calibration  of,  by  means  of 

mercury,  83. 
Butter,  analysis  of,  500. 

determination  of  alkali  required  for 

saponification,  506. 
determination  of  alkali  neutralized 
by  the  soluble  and  insoluble  acids 
in,  508. 
determination  of  insoluble  acids  in, 

502. 

determination  of    iodine    absorbed 
by,  510. 


Butter,  determination  of,  in  milk,  512. 
determination  of  volatile  acids  in,  504. 

Calcium    carbonate    and    ammonium 
chloride,    decomposition    of    sili- 
cates by,  318. 
Calcium  chloride,  drying  of  gases  by, 

339. 

detection  of,  421. 
determination  of,  417. 
determination  of,  as  sulphate,  429. 
determination  of,  as  carbonate,  430. 
determination   of,  as  carbonate   or 

oxide,  430. 
separation  of,  417. 
separation    of,    from    barium    and 

strontium,  417. 

separation  of,  from  magnesium,  441. 
Calcium  oxide,  drying  of   gases  by, 

345. 

Calibrating  cup,  61. 
Calibration  of  burettes  by  means  of 

mercury,  83. 
of  a  eudiometer,  60. 
of  a  thermometer,  45. 
Capillary    depression,    correction    of 

barometer  for,  35. 
Carbon,  determination  of,  in  organic 

compounds,  356. 
determination  of,  combined  in  steel 

colorimetrically,  373. 
determination  of  fixed,  in  coal,  377. 
determination  of  graphitic,  in  iron, 

372. 

determination  of  total,  in  iron  and 
steel  by  oxidation  with  chromic 
acid,  368. 

determination  of  total,  in  iron  and 
steel  by  combustion  in  a  current 
of  oxygen,  366. 

Carbon  dioxide,  an  absorption  appa- 
ratus for,  355. 
Carbonates,  determination  of  carbonic 

acid  in,  326. 

acid,  determination  of,  in  the  pres- 
ence of  neutral  carbonates,  416. 


INDEX 


549 


Carbonates,  "preparation  of  standard 
solutions  of  acids  and  alkalies  by 
means  of,  123. 

Carbonic  acid,  determination  of,  320. 
determination  of,  in  the  air,  332. 
determination  of  total,  in  water,  323. 
determination  of,  in  carbonates,  326. 
separation  of,  from  other  acids,  330. 
filtration  in  atmosphere  free  from, 

330. 
gas,  preparation  of,  free  from  air, 

336. 

volumetric  determination  of,  326. 
volumetric  determination  of  free  and 

semi-combined,  in  water,  320. 
Carius'    method,  of  determining  the 

halogens,  234. 
of  determining  sulphur  in  organic 

compounds,  257. 
Center  of  gravity,  effect  of  location  of, 

upon  sensibility  of  balance,  4. 
of  balance,  position  of,  3. 
Chatard's    method     of     determining 

water  in  silicates,  307. 
Chemical  method  of  determining  molec- 
ular weights,  153. 
Chlorine,  determination  of,  gravimet- 

rically  as  silver  chloride,  209. 
determination  of,  by  Mohr's  method, 

216. 
determination    of,    by     Volhard's 

method,  218. 

Chromic  acid,  determination  of  total 
carbon  in  iron  and  steel  by  oxida- 
tion with,  368. 

iodometric  determination  of,  228. 
Cistern  barometer,  30. 
Citrate-insoluble      phosphoric      acid, 

determination  of,  290. 
Coal,  determination  of  fixed  carbon  in, 

377. 

determination   of  volatile   combus- 
tible matter  in,  376. 
determination  of  fuel  value  of,  375. 
determination  of  water  in,  376. 
Cochineal,  115. 


Colloidal  precipitates,  207. 
Combustion,  of  a  liquid  in  the  open 

tube,  359. 

of  a  solid  in  the  open  tube,  356. 
in  the  closed  tube,  361. 
of  a  compound  containing  sulphur, 

363. 

of  halogen  compounds,  364,  544. 
of  substances  containing  nitrogen  by 

electrical  method,  543. 
of  organic  compounds  by  electrical 

method,  537. 

of  sulphur  compounds,  545. 
Comparison  of  thermometers,  51. 
of  weights  of  balance  with  standard 

weights,  28. 
Cone,  platinum,  201. 
Copper,  electrolytic  determination  of, 

495. 

metallic,  for  organic  analysis,  351. 
Copper  oxide,  for  organic  analysis,  347. 
Copper  sulphate,  drying  of  gases  by, 

345. 
Correction  of  gas  volumes  for  pressure, 

73. 

for  temperature,  74. 
Coulomb,  the,  474. 
Crucible  furnace,  518. 
Crurn's  method  of  determining  nitric 

acid,  265. 
Current,  determination  of  the  strength 

of,  476. 

polarization,  493. 
sources  of,  479. 

Density,  128. 
Density  of  gases,  146. 
determination  of,  by  time  required 

for  diffusion,  149. 
determination  of,  by  weighing  after 

absorption,  148. 

determination  of,  by  weighing  di- 
rectly, 147. 
Density  of  water  between  0°  and  25°, 

80. 
Desiccation,  192. 


550 


QUANTITATIVE  EXERCISES 


Dittmar  and  Robinson's  method  of 
determining  organic  matter  in 
water,  385. 

Drying  of  substances  containing  water 
of  crystallization,  191. 

Dumas'  method,  of  determining  molec- 
ular weights,  158. 
of  determining  nitrogen,  276. 

Dupre"  and  Hake's  method  of  deter- 
mining organic  matter  in  water, 
384. 

Eggertz's  method  of  determining  com- 
bined carbon  in  steel,  373. 

Electric  heating  appliances  for  labora- 
tory use,  517. 

Electric   method,    for  combustion    of 

halogen  compounds,  544. 
for  combustion  of  substances  con- 
taining nitrogen,  543. 
for    combustion    of    organic    com- 
pounds, 537. 

for    combustion    of    sulphur    com- 
pounds, 545. 

Electrodes,  488. 

Electrolysis,  488. 

Elutriation,  304. 

Eudiometer,  calibration  of,  60. 

Evaporation  of  liquids,  186. 

Fahrenheit's  hydrometer,  137. 

Faraday's  laws,  491. 

Fats  and  oils,  acids  found  in,  500. 

Ferrous  ammonium  sulphate,  459. 

Fertilizers,  determination  of  phos- 
phoric acid  in,  288. 

Filter,  Gooch,  201. 
Monroe,  203. 

Filter  pump,  205. 

Filters,  200. 

Filtration,  203. 

Filtration  in  an  atmosphere  free  from 
carbonic  acid,  330. 

Foerster's  modification  of  Kjeldahl's 
method  of  determining  nitrogen, 
275. 


Fortin  barometer,  30. 

Frankland   and  Armstrong's  method 

of  determining  organic  matter  in 

water,  386. 
"Free"  ammonia,  determination  of, 

in  water,  270. 
Freezing-point  method  of  determining 

molecular  weights,  169. 
Fresenius'  method  of  determining  sul- 
phur in  iron,  252. 
Friction,  effect  of,  upon  sensibility  of 

a  balance,  6. 
Fuel  value  of  coal,  determination  of, 

375. 

Furnace,  electric,  for  crucibles,  518. 
tube,  531. 

Gas  analysis,  387. 

Gas,  illuminating,  analysis  of,  401. 

thermometers  filled  with,  51. 
Gases,  absorption  of,  by  liquids,  70. 
density  of,  146. 
determination    of    density    of,    by 

weighing  gases  directly,  147. 
determination    of    density    of,    by 

weighing  after  absorption,  148. 
determination    of    density    of,    by 

time  required  for  diffusion,  149. 
drying  of,  339. 

measurement  of,  over  water,  69. 
passage  of,  through  rubber,  77. 
Gauss,  weighing  by  method  of,  21. 
Gay-Lussac's  method  of  determining 

molecular  weights,  161. 
Gay-Lussac's  volumeter,  138. 
Glass,  composition  of,  187. 
determination  of  specific  gravity  of, 

131. 

tubes,  graduation  of,  94. 
Gooch  filter,  201. 
Graduation  of  glass  tubes,  94. 

of  a  measuring  flask,  81. 
Graphite  stoves,  524. 
Gunning's  modification  of  Kjeldahl's 
method  of  determining  nitrogen, 
274. 


INDEX 


551 


Halogen  compounds,  combustion   of, 

by  electrical  method,  544. 
Halogens,  determination  of,  by  Carius' 

method,  234. 
determination     of,     by     the     lime 

method,  232. 

Halogens,  separation  of,  236. 
separation  of,  by  method  of  Jannasch 

and  Aschoff,  239. 
Hardened  paper,  201. 
Heating  appliances,  electric,  for  labora- 
tory use,  517. 

Hehner's  method  of  determining  insol- 
uble acids  in  fats  and  oils,  502. 
Ilempel,  analysis  of  illuminating  gas 

by  method  of,  401. 
Hofmann's    method    of    determining 

molecular  weights,  162. 
Hot-air  baths,  194. 

Hubl's  method    for   determining  the 
iodine  absorbed  by  fats  and  oils, 
510. 
Hydrochloric   acid,  decomposition  of 

silicates  by,  314. 

preparation  of  a  tenth-normal  solu- 
tion of,  123. 

Hydrofluoric  acid,  separation  of  alka- 
line earths  from,  434. 
decomposition  of  silicates  by,  316. 
Hydrogen,  determination  of,  356,  398. 
determination  of,  by  absorption  with 

palladium,  399. 
determination  of  oxygen  in  air  by 

explosion  with,  392. 
determination  of,  by  explosion  with 

oxygen,  398. 
Hydrogen  peroxide,  determination  of, 

472. 

Hydrogen  sulphide,  separation  of  phos- 
phoric acid  by,  300. 
Hydrometer,  Beaurne's,  141. 
Beck's,  143. 
Fahrenheit's,  137. 
Nicholson's,  138. 
Specific  gravity,  140. 
Twaddell's,  143. 


Hypochlorous  acid,  determination  of, 

by  Wagner's  method,  229. 
determination  of,  by  Penot's  method, 
230. 

Incineration,  377. 

Indicators,  110. 

Iodine,  determination  of  amount  of, 

absorbed  by  fats  and  oils,  510. 
determination     of,    gravimetrically, 

213. 
determination     of,    volumetrically, 

221. 

resublimation  of,  223. 
solution  of,  224. 
lodometric  determination  of  arsenious 

acid,  229. 

determination  of  chromic  acid,  228. 
determination   of  sulphurous  acid, 

226. 

Iron,  determination  of  carbon  in,  366. 
determination   of   graphitic  carbon 

in,  372. 
ferrous  and  ferric,  determination  of, 

in  a  silicate,  464. 

determination  of,  by  potassium  per- 
manganate, 459. 

determination  of  phosphorus  in,  287. 
determination  of  silicon  in,  318. 
electrolytic  determination  of,  497. 
metallic,  determination  of,  461. 
separation  of,  from  aluminium,  466. 

Jannasch's    method    of    determining 

water  in  silicates,  306. 
Jodlbauer's  modification  of  Kjeldahl's 

method  of  determining  nitrogen, 

275. 
Joule,  the,  477. 

Kjeldahl's    method    of    determining 

nitrogen,  272. 

Knife  edges  of  a  balance,  4. 
Koettstorfer's  method  of  determining 

the  alkali  required  for  saponifica- 

tion  of  butter,  506. 


552 


QUANTITATIVE  EXERCISES 


Kubel's     method      for     determining 
organic  matter  in  water,  381. 

Lactometer,  the,  144. 

Latitude,  correction  of  barometer  for, 
35. 

Lead  acetate,  separation  of  phosphoric 
acid  by,  300. 

Lead  chromate  for  organic  analysis,349. 

Lead  oxide,  decomposition  of  silicates 

by,  312. 

determination  of  water  in  silicates 
by  means  of,  307. 

Length  of  beam  of  balance,  5. 

Liebig's  method  of  determining  sul- 
phur in  organic  compounds,  256. 

Liquid  baths,  197. 

Liquids,  bumping  of,  189. 
evaporation  of,  186. 

Litmus,  111. 

Location  of  a  balance,  6. 

Magnesia  mixture,  284. 
Magnesium,  determination  of,  437. 
pyroarseniate,  determination  of  ar- 
senic as,  301. 

separation  of,  from  the  alkalies,  438. 
separation  of,  from  barium,  440. 
separation  of,  from  calcium,  441. 
separation  of,  from  barium,  stron- 
tium, and  calcium,  442. 
separation  of,  from  strontium,  440. 
Manganese,  determination  of,  by  potas- 
sium permanganate,  468. 
reduction  of  permanganic  acid  by 

oxides  of,  470. 

Measurement  of  gases  over  water,  69. 
Measuring  flask,  graduation  of,  81. 
Meniscus,  determination  of  the  value 

of,  66. 

Mercurous  nitrate,  separation  of  phos- 
phoric acid  by,  298. 
Mercury  column  of  thermometer,  cor- 
rection for  exposed  part  of,  50. 
Mercury,  purification  of,  in  calibration 
of  a  eudiometer,  57. 


Metals,  electrolytic  determination  of, 

475. 

Methyl  orange,  114. 
Meyer's  baths,  196. 
Milk,  determination  of  butter  in,  512. 
Mohr,  system  of,  83. 
Mohr's  method  of  determining  silver 

and  chlorine,  216. 

Mohr-Westphal  balance,  determination 
of  specific  gravity  of  liquids  by,  135. 
Moisture  in  air,  correction  of  barom- 
eter for,  38. 
Molecular  weights,   chemical  method 

of  determining,  153. 
determination  of,  150. 
Dumas'  method  of  determining,  158. 
determination    of,    by    Hofmann's 

method,  162. 
determination  of,  by  freezing-point 

method,  169. 

Gay-Lussac's  method  of  determin- 
ing, 161. 

physical  methods  of  determining,  1 57. 
Monroe  filter,  203. 

Morse  and  Burton's  method  of  deter- 
mining the  alkali  neutralized  by 
soluble  and  insoluble  acids,  508. 

Nicholson's  hydrometer,  138. 
Nickel,  electrolytic  determination  of, 

496. 

Nitric  acid,  determination  of,  261. 
Nitrogen,  261. 
combustion  of  substances  containing, 

by  electrical  method,  543. 
organic,  determination  of,  3-85. 
Nitrous    acid,    determination    of,    by 

permanganate,  472. 
determination    of,    by    method    of 
Griess,  Preusse,  andTiemann,  267. 
Normal  solutions,  99. 

Ohm,  the,  474. 
Ohm's  law,  476. 

Organic  compounds,  combustion  of,  by 
electrical  method,  537. 


JXDKX 


553 


Organic  compounds,  determination  of 
carbon  and  hydrogen  in,  356. 

determination  of  halogens  in,  232. 

determination  of  nitrogen  in,  272. 

determination  of  phosphorus  in,  297. 

determination  of  sulphur  in,  256. 
Organic  matter  in  water,  determina- 
tion of,  380. 
Oxalic  acid,  preparation  of  pure,  116. 

preparation  of  a  tenth-normal  solu- 
tion of,  117. 

Oxygen,  apparatus  for  the  purification 
of,  353. 

for  organic  analysis,  353. 

determination  of,  in  air  by  absorption 
with  potassium  pyrogallate,  397. 

determination  of,  by  explosion  with 
hydrogen,  392. 

determination  of,  by  absorption  with 
phosphorus,  395. 

determination  of  hydrogen  by  explo- 
sion with,  398. 

Palladium,  determination  of  hydrogen 
by  absorption  with,  399. 

Paper,  ashless,  200. 
hardened,  201. 

Penot's  method  of  determining  hypo- 
chlorous  acid,  230. 

Permanganate  solution,  preparation  of, 
459. 

Permanganic  acid,  reduction  of,  by 
oxides  of  manganese,  470. 

Pettenkofer's  method  of  determining 
free  and  semi-combined  carbonic 
acid  in  water,  320. 

Phenolphthalein,  113. 

Phosphate  rock,  determination  of 
phosphoric  acid  in,  283. 

Phosphates,  412. 

Phosphoric  acid,  separation  of  alka- 
line earths  from,  433. 
in  phosphate  rock,  determination  of, 

283. 

determination  of,  in  fertilizers,  288. 
determination  of  water-soluble,  289. 


Phosphoric  acid,  determination  of  cit- 
rate-insoluble, 290. 
determination  of  total,  291. 
separation  of,  by  ammonium  molyb- 

date,  298. 

separation   of,   by  ammonium  sul- 
phide, 300. 
separation  of,  by  hydrogen  sulphide, 

300. 

separation  of,  by  lead  acetate,  ,300. 
separation  of,  by  mercurous  nitrate, 

298. 

separation  of,  by  silver  nitrate,  299. 
separation  of,  by  sulphuric  acid,  301. 
separation  of,  by  tin  oxide,  299. 
volumetric  method  for  determina- 
tion of,  292. 
Phosphorus,  283. 
determination  of  oxygen  in  air  by 

absorption  with,  395. 
determination  of,  in  iron,  287. 
determination   of,  in  organic  com- 
pounds, 297. 
Phosphorus  pentoxide,  drying  of  gases 

by,  343. 

Physical  methods  of  determining  mo- 
lecular weights,  157. 
Platinum  cone,  201. 

wire  stoves,  533. 

Potassium,  determination  of,  in  a  sili- 
cate, 413. 

determination  of,  as  double  potas- 
sium-platinum chloride,  407. 
methods  of  determining,  409. 
separation  of,  from  other  substances, 

411. 

separation  of,  from  sodium,  410. 
Potassium  hydroxide,  drying  of  gases 

by,  345. 

preparation  of  a  tenth-normal  alco- 
holic solution  of,  119. 
Potassium  iodide,  solution  of,  224. 
Potassium  permanganate,  determina- 
tion of  iron  by  means  of,  459. 
determination     of     manganese    by 
means  of,  468. 


554 


QUANTITATIVE  EXERCISES 


Potassium  permanganate,  reactions  of, 
454. 

Potassium  pyrogallate,  determination 
of  oxygen  in  air  by  absorption 
with,  397. 

Potassium  tetroxalate,  459. 
standardizing  acids  and  alkalies  by 
means  of,  126. 

Practice  with  balance,  10. 

Precautions  to  be  observed  in  weigh- 
ing, 8. 

Precipitates,  colloidal,  207. 
washing  of,  205. 

Precipitation,  198. 

Preparation  of  pure  oxalic  acid,  116. 

Pressure,  correction  of  gas  volumes 
for,  73. 

Purification  of  mercury  in  calibration 

of  a  eudiometer,  57. 
of  substances,  186. 

Pycnometer,  determination  of  specific 
gravity  of  liquids  with,  133. 

Pycnometers,  55. 

Pyrophosphate,  determination  of  man- 
ganese as,  471. 

Recrystallization,  190. 
•  Reichert's  method  of  determining  vol- 
atile acids  in  butter,  504. 
Reichert's  thermostat,  194. 
Resublimation  of  iodine,  223. 
Rheostats,  490. 
Rubber,  passage  of  gases  through,  77. 

Sauer's   method  of  determining  sul- 
phur in  organic  compounds,  258. 
Schulze's  method  of  determining  or- 
ganic matter  in  water,  381. 
Sensibility  of  balance,  4. 
determination  of,  13. 
effect  of  friction  upon,  6. 
effect  on,  of  location  of  center  of 

gravity,  4. 

Sifting  of  silicates,  303. 
Silica,  determination  of,  310. 
separation  of,  from  other  substances, 
319. 


Silicates,   decomposition  of,  by  acids 

under  pressure,  315. 
decomposition  of,  with  an  alkaline 

carbonate,  310. 
decomposition    of,    by    ammonium 

chloride  and  calcium  carbonate, 

318. 

decomposition  of,  by  barium  hydrox- 
ide and  carbonate,  317. 
decomposition  of,  by  bismuth  oxide, 

314. 
decomposition   of,  by  hydrochloric 

acid,  314. 
decomposition  of,   by   hydrofluoric 

acid,  316. 

decomposition  of,  by  lead  oxide,  312. 
determination  of  ferrous  and  ferric 

iron  in,  464. 
determination    of    potassium    and 

sodium  in,  413. 
determination  of  water  in,  by  borax 

method,  307. 
determination  of  water  in,  by  Cha- 

tard's  method,  307. 
determination  of  water  in,  by  Jan- 

nasch's  method,  306. 
determination  of  water  in,  by  means 

of  lead  oxide,  307. 
determination  of  water  in,  by  Sip- 

coecz's  method,  305. 
Silicon,  determination  of,  in  iron,  318. 
Silver,    determination    of,    gravimet- 

rically,  209. 
determination  of,  by  Mohr's  method, 

216. 
determination     of,     by    Volhard's 

method,  218. 
Silver  coin,  determination  of  specific 

gravity  of,  130. 
Silver  nitrate,  separation  of  phosphoric 

acid  by,  299. 
Sipcoecz's    method    of    determining 

water  in  silicates,  305. 
Siphon  barometer,  31. 
Sodium,  determination  of,  410. 
determination  of,  in  a  silicate,  413. 


555 


Sodium,  separation  of,  from  other  sub- 
stances, 411. 

separation  of,  from  potassium,  410. 
Sodium  carbonate,  standardizing  acids 

and  alkalies  by  means  of,  127. 
Solutions,  standard  and  normal,  99. 
Specific  gravity,  128. 
bulbs,  145. 

determination  of,  by  dilution,  145. 
of  glass  in  fragments,  determination 

of,  131. 

hydrometers,  140. 
of  liquids,  determination  of,  by  Mohr- 

Westphal  balance,  135. 
determination  of,  with  pycnometer, 

133. 
determination  of,  by   weighing  an 

object  in  two  liquids,  134. 
of  a  silver  coin,  determination  of,  130. 
Standard  solutions,  99. 
of  acids  and  alkalies,  preparation  of, 

by  means  of  carbonates,  123. 
preparation  of,  by  means  of  oxalic 

acid,  117. 
Standard  weight,  comparison  of  weights 

of  balance  with,  28. 
Standardizing  acids  and  alkalies,  by 
means  of  potassium  tetroxalate, 
126. 

by  means  of  sodium  carbonate,  127. 
by  means  of  sulphuric  acid  derived 

from  copper  sulphate,  127. 
Starch,  preparation  of  paste  of,  224. 
Steel,  determination  of  carbon  in,  366. 
Stoeckmann-Fresenius  method  of  de- 
termining phosphorus  in  iron,  287. 
Stoves,  graphite,  524. 
platinum  wire,  533. 
Strontium,  detection  of,  421. 
determination  of,  417. 
determination  of,  as  sulphate,  427. 
determination  of,  as  carbonate,  428. 
separation  of,  417. 
separation  of,  from  barium  and  cal- 
cium, 417. 
separation  of,  from  magnesium,  440. 


Sulphates,  412. 

Sulphur,  combustion  of  a  compound 
containing,  363,  545. 

determination  of,  in  iron,  by  Fre- 
senius'  method,  252. 

determination   of,  in   organic  com- 
pounds, 256. 

determination  of,  in  sulphides,  247. 
Sulphuric  acid   derived  from   copper 
sulphate,  standardizing  acids  and 
alkalies  by  means  of,  127. 

determination  of,  in  barium  sulphate, 
244. 

drying  of  gases  by,  342. 

separation  of  alkaline  earths  from, 
432. 

separation  of  phosphoric  acid  by,  301. 

iodometric  determination  of,  226. 

Tares,  weighing  by,  25. 
Temperature,  correction  of  barometer 

for,  33. 

correction  of  gas  volumes  for,  74. 
determination  of,  by  substances  of 

known  melting  points,  54. 
Thermometer,  calibration  of,  45. 
correction  of  exposed  part  of  mercury 

column  in,  50. 
determination  of  boiling  point  of, 

43. 

determination  of  zero  point  of,  38. 
Thermometers,  alcohol,  54. 
air,  52. 

Beckmann,  52. 
comparison  of,  51. 
filled  with  gas,  51. 
toluene,  54. 

Thermostat,  Reichert's,  194. 
Tidy's  method  of  determining  organic 

matter  in  water,  382. 
Tiemann-Schulze's  method  of  deter- 
mining nitric  acid,  261. 
Time  of  vibration  in  weighing,  deter- 
mination of,  10. 

Tin   oxide,  separation  '  of  phosphoric 
acid  by,  299. 


556 


QUANTITATIVE  EXERCISES 


Toluene  thermometers,  54. 

Total  phosphoric  acid,  determination 

of,  291. 

Tropseolin,  115. 
Tube  furnace,  531. 
Twaddell's  hydrometer,  143. 

Vapor,  determination  of  volume  of,  by 

displacement,  165. 
determination  of  volume  of,  by  pres- 
sure, 164. 

Varrentrapp  and  Will's  method  of  de- 
termining nitrogen,  281. 

Verbeck  and  Peckholdt's  balance,  15. 

Volatile   acids,   determination  of,   in 
butter,  504. 

Volhard's  method  of  determining  chlo- 
rine and  silver,  218. 

Volt,  the,  475. 

Voltage,  determination  of,  477. 

Volume  of  water  between  0°  and  25°, 
80. 

Volumeter,  Gay-Lussac's,  138. 

Wagner's  method  of  determining  hy- 

pochlorous  acid,  229. 
Wanklyn,     Chapman,     and     Smith's 

method  of  determining  free  and 

albuminoid  ammonia  in  water,  270. 
Washing  of  precipitates,  205. 
Water,  absorption  apparatus  for,  354. 
density  and  volume  of,  between  0° 

and  25°,  80. 

determination  of,  in  coal,  376. 
determination  of  hardness  of,  450. 
determination  of  organic  matter  in, 

380. 
determination  of  organic  matter  in, 

by  method  of  Dupre"  and  Hake, 

384. 
determination  of  organic  matter  in, 

by  method  of  Dittmar  and  Robin- 
son, 385. 
determination  of  organic  matter  in, 

by    method    of    Frankland    and 

Armstrong,  386.     ' 


Water,  determination  of  organic  mat- 
ter in,  by  method  of  Kubel,  381. 
determination  of  organic  matter  in, 

by  method  of  Schulze,  381. 
determination  of  organic  matter  in, 

by  method  of  Tidy,  382. 
determination  of  organic  matter  in, 
by  method  of  Wolff,  Degener,  and 
Hergfeld,  383. 

determination   of  permanent  hard- 
ness of,  452. 

determination  of,  in  silicates,  304. 
determination  of  total  hardness  of, 

451. 

of   crystallization,    drying   of   sub- 
stances containing,  191. 
hardness  of,  448. 

Water-soluble  phosphoric  acid,  deter- 
mination of,  289. 
Watt,  the,  478. 
Weighing,  correction  for  air  displaced 

in,  22. 
determination  of  time  of  vibration 

in,  10. 

determination  of  zero  points  in,  11. 
by  method  of  Borda,  20. 
by  method  of  Gauss,  21. 
by  usual  method,  19. 
practice  in,  19. 

precautions  to  be  observed  in,  8. 
by  tares,  25. 

Weight  of  beam  of  balance,  5. 
Weights,  correction  of,  26. 

materials  for,  28. 

Wilfarth's  modification  of  Kjeldahl's 
method  of  determining  nitrogen, 
273. 

Wolff's  method  of  determining  organic 
matter  in  water,  383. 

Zero  point  of  thermometer,  determina- 
tion of,  38. 

Zero  points,  determination  of,  in  weigh- 
ing, 11. 

Zinc,  electrolytic  determination  of, 
498. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


DEC   3    1947 


DEC    20  194? 


MAY    13  1948 


REC'D  LI 

DEC  9     19! 


v G 'D 


27Mar'58J| 
REC'D  (  O 

APR    ! 


REC'D 

OCT  1 8  195! 


LD  21-100m-9,'47(A5702sl6>«6 


e_  I  GO 


